a 2002 cartoon network promo, which you can find on youtube, is what I believe sparked this pairing. This is one of several images that are found online and are very well done and appropriate for a general audience.
Eh, Semi-Canon at least. Crossover episode, she had the hots for him. At least 2 bumper commercials where it's clear they are dating. He calls out to her at the end of the Johnny Goes to Bollywood movie. There's something there, certainly more than you find in other Velma ships (shots fired)