Nicotine base is simply nicotine in unflavored base. The nicotine base can be purchased as VG, PG or a blend of both. Flavoring and thinners are added to the base. Most flavorings are PG, VG or alcohol based. E-liquid calculators can help you keep track of the mixture.
If you want a high percentage VG juice it is best to start with VG base nicotine, dilute with VG than add VG based flavoring and any thinners you may want.
The best place to find information about DIY juice is right here at the DIY forum. There are lots of informative stickies to read.
DIY E-Liquid