Important Guideline For Buying Vitamins

Whats is the guideline in buying vitamins?

Right now I only know of two guidelines.

1) make sure the vitamins that you are going to purchase are manufactured according to gmp (good manufacturing practices for pharmaceuticals.

2) make sure that the pills that you are taking are produced under usp (us. Pharmacopoeia standards) usp ensure that the pills will dissolve properly

please correct me if my information is wrong
what other guidelines are there to ensure that we are eating high quality vitamins and protect consumers like us?