I'm not quite sure I understand what you are saying. If you are at 30mL already and it is 50/50 and you dont want to go above 30mL, you can't add any VG without getting rid of some of what is already in the bottle. If that is what you are talking about, I will try to be of some help. At the for 30mL of 70vg/30pg, you want 21mL VG and 9mL PG. You currently have 15mL of VG and 15mL of PG in the bottle. If you want to get that down, you are going to have to take out 12mL overall(that will remove 6mL of PG and 6mL of VG, leaving you with 18ml, consisting of 9mL each of VG and PG), and then add 12mL of your pure VG.
Keep in mind, this is going to reduce the amount of flavor by 40%, since only 60% of the mix will be juice that was previously flavored.