First thing to let you know is that by adding unflavored VG to a premixed
juice is going to change the overall flavor itself. More than likely it will mute the strength of the flavor and how much depends on how much VG you add.
I do not know What size bottle you have or what ratio you want so I will use 10ml bottle and 50/50 end result for ease of explaination. If you want a higer VG ratio just adjust the amount you add accordingly.
10ml 60pg/40vg = 6ml.pg/4ml.vg
So to get a 50/50 mix just add 2ml. of VG giving you a total of 12ml of
juice with 6ml of pg and 6ml of vg.