The reason I throw enhancers into quotes is because it's kind of a misleading title. When people talk about these flavor enhancers they really are only talking about single aroma compounds. These aroma compounds just make small changes to your recipes due to the nature of the compound. Also, if used incorrectly will do anything but enhance the flavor of your mix. So instead I like to call them "tools" because that's just what they are. they have a specific purpose for a specific job to aid you in getting the result you're looking for.
Ethyl Maltol & Sucralose
These compounds are sweeteners. These sweeteners are both wildly different from eachother and work completely different as well. So don't think these compounds can be substituted for eachother. Ethyl Maltol is a sweetener that inherently sweetens certain aromas. This means if you have a strawberry flavoring you think would work better in your recipe if that flavor itself was a touch sweeter, using ethyl maltol will help you. Sucralose on the other hand is an overall sweetener. This is akin to putting sugar into your juice. So sucralose adds sweetness on top of your entire mix. This may be confusing at first, but after using these sweeteners you'll understand very quickly what I mean. A great way to use Ethyl Maltol is when you have a recipe where some ingredients you are using could use some sweetness. So if a strawberry banana recipe you have is kind of lacking sweetness by the strawberry because of the thickness of the banana, and you don't want to add an overall sweet flavor to the recipe, then a touch of Ethyl Maltol could be used to bring forward the sweetness just a bit. If you wanted the entire mix to have a sweeter flavor, where the sweetness is felt along the entire palate, then a touch of sucralose would be better. Ethyl Maltol inherently sweetens where sucralose completely sweetens. That's the best way I can describe the difference between the two. Now these sweeteners, if used incorrectly, can completely mute and dumb down your recipes. The reason a lot of mixers can't stand commercial juice is because of the amount of sweetener they add in their mixes. The reason they do it is to hide any imbalances or imperfections in the recipe by covering it up with a sweet taste. It's the same reason a lot of American food is so sweet. People attribute sweet flavor to being good, but it's simply not the case. Nonetheless these are two VERY popular "enhancers" and once you start mixing your own recipes you should reserve a spot for them. Sweeteners can make or break your recipes so be smart when you use them.
This is another popular "enhancer". Acetyl Pyrazine is a compound that imparts a bready, grainy, flavor to recipes while also bringing out a more grainy texture. So bakery's, cookies, cereals, any recipe with some sort of "grainy" texture benefit from the use of Acetyl Pyrazine. The best way to use this tool is say you have a cereal recipe and the distinction of the actual cereal flavor isn't as prominent as you'd like it. Adding around a half a percent of Acetyl Pyrazine can bring forward that grainy texture in that cereal note, bringing more attention to that part of the mix. Same goes for a pie where you want the grainy crust to come out a bit more. Think about using Acetyl Pyrazine that way, it's the best way to use the tool. Acetyl Pyrazine can also smooth out and bring forward bitter notes in dark flavors like chocolates and coffees. Using AP this way is a bit more tricky but it can help tremendously. Again, if you use too much of this flavor you will end up adding too much of that bready and grainy flavor in your mix which, trust me, is no fun. So be conservative when using this tool. This also can make or break a recipe. I recommend more advanced users to get this ingredient. Those with a good idea of the structure of their flavorings and who know exactly what flavor profiles they want to portray.
Malic & Citric Acid
These two compounds are for more of the advanced mixers to use. What Malic and Citric acid do is allow certain aromas to have a little more of that tart and sour flavor come out of certain ingredients. Now adding Malic and Citric isn't going to give you this wonderfully tart and sour experience in your vape, but they do it in a more subtle and light way. So say you have a lemonade flavor that you think would taste better if that lemonade gave more of a tart flavor than it already has, than a touch of Malic or Citric will aid you. Citric acid is a tool that not many people use because of its muting properties. It's way too finicky to be practical in any mix. This is mainly due to the amount of citric that's already in many of the tart and sour flavors. Malic acid is the same way, but doesn't have as much muting power as citric does. TFA Sour is the exact same thing as Malic Acid, so you can just pick up that if you want a tool that will give you those profiles. These should exclusively be used in fruit and candy mixes where having a tarter flavor will benefit the recipe. In anything else such as bakeries or desserts, it will just ruin the mix. Again, use very conservatively.
Triacetin, MTS Vape Wizard, and TFA Smooth all generally do the same thing. They add somewhat of a thicker and smoother feeling to your mixes. Just like the enhancers before it, adding these compounds will not give you these extremely thick and heavy recipes with a velvety texture. These ingredients really only give a very subtle change. Almost to the point where it's not even worth using because of how specific they are to use. I generally stay away from adding extra Triacetin. By itself all it really does it distract the palate away from the profile you want to give attention to. But TFA Smooth and MTS Vape Wizard, which have triacetin in them along with other compounds, do help in some situations. The best way to use this tool is when you want a more "blended" flavor. So say you have a blueberry banana smoothie recipe where you can completely tell the differences between all the flavors, but you want those ingredients to taste more if they were one flavor. Adding MTS VW or TFA Smooth will help you out a bit. It's akin to throwing all your ingredients into a blender rather than layering them in. So using these tools can kill any kind of subtleties, layering, and complexity your mixes have, which may be the flavor you're going for. Also, using these in tobaccos (MTS Vape Wizard was made FOR tobacco) will really help give that "smokey" feeling on the palate by adding some weight and further aiding the authenticity of your recipe. These are definitely for more advanced mixers and honestly, you can do without them.
What Vanillin is, is in the name. It's a single aroma taken straight from the vanilla ingredient. The best way I can describe this tool is thinking it's like vanilla extract. It adds an overall vanilla flavor to your recipes, just like how vanilla extract adds an overall vanilla flavor to anything you put it in. The difference between vanillin and a vanilla flavoring is that you can't layer in the vanillin, where you can with a vanilla flavoring. So say you have a raspberry ice cream flavor and you want to add a touch of vanilla either on the back or in the front, well then vanillin is not what you want to use. What the vanillin will do is give your entire mix a vanilla property that you'll taste throughout the entire vape. It's a specific ingredient, but a useful one when you do need it. If you have a macaron flavor and you want to give it an overall vanilla flavor profile to help add in some intricacies, then vanillin is a great use. Remember to use it conservatively. Vanillin usually comes at a 10% concentration and is best used very low.
Menthol & Koolada
These two are the only "ingredients" of the bunch meaning you need to have it already in your head before you go to make your recipe, that you want these ingredients in there. Everyone knows what menthol is. It's a minty flavoring with an extremely cool and refreshing sensation. Koolada does exactly what menthol does, but doesn't really carry that minty flavor menthol has. These two are also the ingredients that will give you the most notable effect on your recipe, where all the other ones are very subtle. Adding even a drop or two of menthol or koolada will completely change the flavor of your mixes. These compounds are so fun to use. That cooling sensation does so many different things to certain flavors, and the possibilities of flavor profiles is endless when using them. Menthol usually comes in a PG base, but you can also purchase crystals if you want more control of your concentrations. Koolada I believe just comes in a base. Because of the nature of these ingredients I will leave it up to you how to use them because the surprises that come from them makes using them a blast. Just be conservative and you'll be just fine. (BTW, Raspberry Vanilla Ice Cream with a touch of menthol is excellent)
And for the last "enhancer" we have Champagne. What is really just a flavoring, I like to think of it as an enhancer. What it really does is add a slight carbonated effect to your recipes. So using this is best in beverage, cocktail, and soda type flavors. Because it is a flavoring, you need to take in account the flavors that come with it which is a slightly acid, white grape with light floral notes. So when using this to give that carbonated effect, try to work your base recipe around those notes to fully utilize its properties. I recommend using around 2-3% but the concentration is really based on the situation. Again, these are subtle flavor changes so adding this will not give you this extremely fizzy and carbonated effect. But still, this is a great ingredient to add to your arsenal once you have a good understanding of the flavorings you own.