Are there available vendors that sell the concentrated e-juice flavors? On occasion, I'd like to add a few drops of, say peppermint, to a mix that I already have.
I think you are looking into DIY (Do it Yourself) juices by chance? Before you get into that process I say use the DIY Baby Step 0.5 method. Maybe mix two or more liquids into a tank at a time of pre bought stuff at first? I really want to venture into the DIY process but am going to wait until my stockpile dwindles a bit before going that route. For now mixing pre made juices is the extent I have done.
As for a peppermint/mint I add this to a lot of my juices
Grasshopper - Mom and Pop Vapor Shop 10-25% at most is needed for my tastes. I add it to tobacco, watermelon, redbull, caramel, vanialla, etc type flavors all the time. Whenever I get a minty type mood. It doesn't take a whole lot to add to get that minty flavor so maybe start off small. Only reason I single out that vendor besides the fact that I like their grass hopper is they are one of the few sites I have found that offer concentrated versions of all of their flavors for DIY purposes
Concentrated Flavor - Mom and Pop Vapor Shop. If you find a flavor that you like you should be able to recreate that with your own pg/vg/nic and their flavors concentrate.
I don't like to push vendors in this sampler thread but I have also found their
Pchela's Famous 5 Layer - Mom and Pop Vapor Shop to be very good and use it often. It's approaching the $0.50 cents per ml limit for me though even when ordered in quanity so it's reaching my max amount of what I want to spend.
Your other option is to look into FDA approved mint type flavors and add a few drops. Your going to have to go to DIY subforum for recommendations on that...but I'm guessing even something like
Menthol Crystals - E Cig eGo, E juice, Mods, Cartomizers, Liquid Nicotine, DIY Flavorings you could add just a little bit to your tank to get you that mint/menthol type flavor. That's probably more economically than mixing the pre bought stuff but have just found for now it's easier to add 25% mint flavor of a juice and 75% of some other juice into a 2-3ml tank. Heck if you don't like it just vape through it. If you were to make a 30-50ml and hate it that's another story. Why I've kind of stuck with the baby step 0.5 DIY method at least for now.