If they are indeed one shots and shake and vape, you can just start small depending on how much "pop" you think you might need. I might start at 2%, seems a reasonable starting point, since your juice is already flavored. Or even 1% as it should be easy to add more (I have found in my flavoring adventures that "add more" is infinitely easier than "dilute down."
Starting out that way should give you a good sense of what is being "added" and also because everyone tastes flavors differently, so if you start low and work your way up, you will eventually reach that "good flavor" point for you. Once you have a sense of how much flavor you need, you may then be able to add a bit more for some of your later mixes (with the caveat that depending on the flavor or manufacturer, things may be slightly different.
I wish I had a less "tedious" answer and could just say "Well ,5% across the board!" but flavoring just doesn't work that way. Letitia gave you some good options for "starting point" on flavors, but keep in mind most of what you read about your flavors may "help" determine a starting point, but when you are adding to already flavored juices I'm afraid some "sure bets" are off, etc.
Anna