Ray J -- Kim Kardashian's Vajayjay Gave Me a Huge Payday

Ray J has a knack for making money when Kim Kardashian gets naked ... and he hit the jackpot again with last week's nude photos.

The photos triggered renewed interest in "Kim K Superstar," and our sources at Vivid Entertainment say sales of the video have dramatically spiked ... so much so, Ray J will pocket $50K from the last week alone ... way up from what's he's been getting.

Ray has been getting around $90K every 3 months, so this is a real ass-tronomical increase.

Steve Hirsch, president of Vivid Entertainment, tells TMZ, the most recent spike makes "Kim K Superstar" the biggest selling celebrity sex tape of all time.

Until last week, the title holders were Pam Anderson and Tommy Lee.