As an exemple; If it is a 10ml bottle, and it now has 4ml /6ml of pg/vg. You want this 4ml PG = 20% so 80% VG = 80/20 * 4ml = 16ml VG .
You have 6ml vg in your present 10ml bottle. You would have to add 10ml of vg thus making it 4ml/16ml of pg/vg . This is a ratio of 20/80. But you then doubled the volume of the un-flavored base to the same flavor content, ( keep in mind that pg/vg may not react the same to flavors).
Obviously it is not the way to go to maintain the same flavor or nic levels
You may be a candidate for DIY .