Just an update, I haven't tried the method you mentioned since I was already finishing up the original method. Ended up with 15ml of flavoring. Mixed 3% into 50/50 pg/vg and after a hot bath ultrasonic session, couldn't really taste anything. Bumped it up to 6%, another hot bath/ultrasonic and I could taste some, repeated again for 9% which seemed just about there in strength, ended up at 10% in the end.
Not really digging it so far, it doesn't taste like my coffee if I brewed it or pulled an espresso with it, it tastes like coffee flavored candy which I hate (note: I avoid coffee flavored anything like a plague, probably the coffee snobbery at work). Didn't feel like mixing a second batch, so I just took that and added 2% of bavarian cream, caramel, and sweet cream respectively figuring I may as well try turning it into a dessert type juice and try covering up the coffee flavor. Day later, still not digging it; either needs time to age or I simply don't like coffee "flavor" vapor anymore than coffee flavored food. I guess I shouldn't be surprised, I'm mainly a straight ristretto shot drinker and can't drink coffee if it's got any flavoring or sweetener added to it (though I do enjoy my cappuccinos/lattes that get sweetened by milk). Considering that vaping straight vg/pg is already sweet on it's own, not much that can be done.
For now this is going on the backburners, got other recipes to work on this week when my latest shipment arrives. I'll take the juice I made yesterday and guinea-pig my friends on thanksgiving and see what they think. They seem to enjoy my various flavor concoctions far more than I do.