I use vanillin crystals and mixing is a matter of taste. Most places recommend 10%, but I've mixed it at higher ratios. The flavor can really come
through, or be a subtle taste. If you really want a strong vanilla taste, add vanillin crystals to PG and shake it aggressively, add more and repeat until cloudy. The excess will settle in the bottom of the bottle. If there are no crystals on the bottom after resting, you could add a bit more.
One note of caution: If you choose to saturate the PG or Distilled Water and it settles on the bottom, use a dropper or syringe and remove the liquid above the residue, then add more PG and shake the residue. If you just leave it, after a few days it will settle hard and become insoluble flakes.