I think that, at this point, Vegeta has reached at least Namek Goku's level and, considering
his fighting skills, he can win against 100% Mecha Freeza. He loses if King Cold joins though.
Doesn't Vegeta says the SSJ is the only thing between him and Goku? I think this implies Vegeta ~ Namek Goku, so it could go both ways, since Vegeta is a better fighter but has a strain. I'm willing to give it to Vegeta, though. He wouldn't mind to use all his energy in a souped up blast.