To the OP, a lot of it is trial and error so I would make several bottles of the mix using different percentages (keep detailed notes) and see which one comes out closer to the flavor your looking for. Seeing that you used 15% cheesecake reminds me of when I first started trying to make a cookie tasting vape I used a high % of the cookie flavoring but found that lowering it to around 5-7% helped it taste more like cookie. About the vendor juices, I think the amount of flavoring varies so much its hard to say. I've heard about a certain vendor juice that uses around 50% flavoring, sounds crazy ha. There's also things like sweetener, AP, EM, apple cider vinegar, distilled water etc that might be used by vendors too in specific juices. Sometimes you get lucky with a juice the first time you make it and other (most) times it takes a lot of tries with a lot of different variables to get it right. Sorry I couldn't be more help, I hope you find the flavor combo your after!
This is how I approach it as well. I will sometimes make as many as 11-12 trials (keeping careful notes) before I settle on a decent base recipe. And I go on to tweak it from there. I do make small test batches, and I have some special liquid measuring tools that allow me to do this accurately, so it may not be the way to go for everyone, although if you are careful with measurements you could use drops of flavor in a similar way.
Another thing I do is once I get a decent base recipe, I will make up a fairly large size batch of that and divide it up into smaller batches. Then I will add different 'accent flavors', sweeteners, or 'enhancers' in different proportions to each smaller portion, to see what else works to give it more complexity. It's amazing how many distinctly different liquids you can make by adding even fractions of a percent of different flavors to the same base recipe.
I don't do this all in one day. I continue tweaking and playing with recipes for weeks and even months. Some of my trials are keepers and some get abandoned quickly.
