Can I just paint over my existing vinyl wrapped MDF kitchen cabinets?
Realistically, no if the vinyl has held up well so far, but it also shouldn't be too much work to peel off the vinyl to prep for refinishing. That said, depending on the cabinets, you could have particle board or MDF underneath, which you wouldn't want directly exposed for painting due to porosity. In that case, lightly scuffing the vinyl with any abrasive, priming, then painting should suffice to give a good coating.

The only real concern I know of with painting over vinyl is the potential for bubbling or warping issues with the vinyl transferring through the painted surface. Like I said, though, if the vinyl has held up well for a good amount of time, I can't foresee any of those issues happening easily.