Justin Bieber Video Girl Cailin Russo -- Swapping Spit with Justin Scored Me a Girl-on-Girl Gig

The video vixen who played tonsil hockey with Justin Bieber has graduated ... from groping the Biebs to fondling supermodel Nina Agdal in a bikini.

TMZ has learned Cailin Russo recently signed a modeling deal with swimwear giant Op, and her first photo shoot was the definition of a win-win (for the world) -- since she's practically attached at the hip with S.I. swimsuit babe Agdal.

Op honchos told us Cailin wasn't on their radar until they saw her in Bieber's "All That Matters" video -- so, uhh ... way to go Justin.

Flashing her boobs probably didn't hurt Cailin's career either.