Probably a vendor who sells through gas stations, bodegas, smoke shops etc ...
From the brands we are familiar with, probably a vendor like five pawns or suicide bunny or space jam who sell primarily through B&M's ( and through sites like Giant Vapes and Ejuices.com ).
Exclusive on-line retailers, Mount Baker Vapor, Alien Visions or Johnson Creek maybe ?
And then you have vendors like Halo and Mountain Oak Vapor who have a strong presence both on-line and through B&M's.
Good question op, not sure anyone knows the answer. What i would guess though is that the the vendor with the largest sales doesn't even own 5 % of the market.