You can add glycerine or PG but if you do 50/50 you loose most if not all the flavor. If you have vanilla flavor mix it with the glycerine or PG than combine the two.
Yes, your math is right. I have a suggestion since you are trying to stretch out a vanilla
juice. Try to just dilute a little bit, 50:50 VG:vanilla. If it doesn't have enough flavor, you can certainly add a few drops of vanilla flavoring...or whatever you like to change its direction if you want.
However, if you are like me and always looking for that great vanilla flavor, try this: instead of pure VG to dilute the juice, use Frontiers Vanilla Flavor, which is a very mild vanilla extract in VG. Its like diluting with vanilla flavored VG rather than pure VG. You may need to add a little vodka or distilled water to think it if its too thick, but I do this with my DIY vanilla juices and its quite good.
Vape on!