Hello, there! I'm making a random and rare appearance.
It needs to be considered that Sanji wasn't fully healthy for this fight, as is the OP's point. Comparing him to Usopp doesn't matter, as Usopp didn't take on an attack after being hit by Caesar, and especially not by someone of Vergo's caliber.
I'm going out on a very small limb and say Sanji has blackened CoA. Even Bellamy has it, so it's not really a reach. Problem is, it's kind of hard to tell if Sanji is using it, since he always wears black pants

But we know he didn't go all out against Vergo, since he didn't use any of his strongest attacks, nor did he use DJ on both legs.
Seeing what Zoro did this weekend, Sanji can't be that far off from his level. They've been comparable for a long time. But I don't think Sanji would defeat Vergo. I think even Law might've still lost to him if Vergo didn't decide to go head-on against his attack. That's something that Vergo did. Instead of trying to dodge attacks, he either blocked them or tanked them. So, while I think Sanji would give him a good fight, I believe he would still lose.
But I could be wrong. We haven't seen Sanji's true level yet. We need to see him in a big fight, like against Pekoms or Tamago. Once we see him in full action, it will be easier to judge this fight. Or make it even harder........ But we will probably get feats like Zoro, where we will see something closer to his true strength.