VAM is an acronym for the authors of the Comprehensive Catalog and Encyclopedia of Morgan and Peace Dollars, Leroy Van Allen and A. George Mallis. In their book, they identified and assigned a VAM number for each of the different die pairs used to strike Morgan and Peace dollars. In all there are over 3,000 different VAM numbers.
Most of the VAMs are not significant and do not bring a premium over the common variety. The varieties that PCGS will do have been identified by variety collectors as worthy of specific recognition. They are listed in: The Top 100 Morgan Dollar Varieties: The VAM Keys; SSDC Official Guide to the Hot 50 Morgan Dollar Varieties; The Official Guide to the Top 50 Peace Dollars; The 1878 Morgan Dollar 8-TF Attribution Guide; and the VAM book, Comprehensive Catalog and Encyclopedia of Morgan and Peace Dollars.