distilled water is just fine. don't has to be Evian

de percentage I think depends on what is your final PG/VG ratio goal to achieve...
It depends on how much final aroma concentration you'll have together with the additives (12-18%), depends on the PG/VG concentration of the base where you mix it in...
The pure VG that you keep at home I thinks don't have to contain any water (0%).
So it keeps the viscosity, remains dense, and you can add up the percenteage to equilibrate the final outcome of every batch you DIY.
I've heard that exceeding 10% is not a good thing. It'll be thin and washy...
But I'm a VG fan, so don't listen to me

(The PG in the aromas and the water in the additives does humify my liquid at a sufficient degree!)