If you're diluting down a mix with plain PG/VG and don't want to lose too much flavour then PG is the best thing to use. VG will 'muffle' the flavour a little more but give more vapour. PG will give less vapour but carry more flavour.
PG is thin and will thin out the resulting e-liquid a little, it will also make it wick better and you'll use more of the e-liquid when you vape. VG will thicken it a little.
If you have both PG & VG then I'd mix up a plain bottle at 70PG/30VG (which is the ratio that most commercial juice uses) and dilute it with that. Just do a small amount first so that you don't make all of your stuff into something you don't like.
Another easy way to dilute down is to get a bottle that's half the strength and mix the 2 together 50/50.