Don't add too much or you will weaken the taste of your
juice. I would use a small bottle and not do all of it at the same time - you can't undo it. Start with adding 5% and see if that gives you enough vapor, it might.
I added too much sweetener to an almost full bottle and after it sat for a few days and ripened was too sweet to
vape. VG doesn't taste very good if you add too much and I think 20% unflavored VG is too much.
Afterthought: After thinking about this I may be wrong since it's AC you're adding VG to. The creamier flavors tend to lose their flavor with unflavored more than the AC does.