Water is not necessarily required/needed. When mixes have a majority of VG, they have a tendency to be very thick. Thick liquids may not wick as well in some atomizers. Therefore, mixers may add water to thin the VG. I personally never went over 5% water, but this is up to you in your final mix.
As far as replacing PG with water, be careful of the percentage of water you use. Water does not vaporize as readily nor does it make good vapor. You could just as easily mix you VG with 5 or so percent of water to thin it down, and use this in place of the PG, which would basically make a 100% VG mix (minus your percentage of water).
Don't know about your flavorings, but most I purchase are PG based. These will naturally add PG to your mix.
Some people don't like PG, some don't like high VG. PG sometimes causes reactions, VG sometimes causes phlegm.
Fun awaits you. No matter what, things will be OK.