we havnt seen vexx yet so v dont know the extent of his abilities. also there is a possibilty that because he is an original appoplaxian he might be more aware of his abilities and he has been prcticing them for years so FOR NOW i say vexx.
It's a draw! The DNA signatures in the Omnitrix are samples from the superiority of that species, so Rath would be the ideal winner. Although, Vexx has been an Appoplexian much longer than Ben, and for all we know Ben hasn't even come close to Raths capabilities. Vexx also appears to be more ruthless, giving him another advantage.