I do think there is a good response to Vlastos' objection. Here I'll explain the objection more fully. In a later post I'll give what I regard as a sufficient reply.
Recall again the argument (Gorgias 474c-475c). Consider a case where A wrongs B:
1. A's wronging B is more shameful than B's being wronged by A.Vlastos objects that, because 'painful' is a relative term (painful to whom?), 2. contains a hidden qualification. The qualification that should be supplied is clear from the sorts of examples Socrates uses to gain Polus' assent to 2. What Socrates insists upon, and what Polus acknowledges, is that noble things (kala) are pleasant when seen, heard, or contemplated, and that shameful or ignoble things (aischra) are painful when seen, heard, or contemplated. The premise should therefore read:
2. What is shameful is so on account of its being painful or bad.
3. What is more shameful is so on account of its being more painful or worse.
4. A's wronging B is not more painful than B's being wronged by A.
5. Thus, A's wronging B is worse than B's being wronged by A.
6. Thus, generally, it is worse to do wrong than to suffer it.
2'. What is shameful is so on account of its being painful to those who see, hear or contemplate it, or bad.And 3. should accordingly read:
3'. What is more shameful is so on account of its being more painful to those who see, hear or contemplate it, or bad.But then 4. becomes
4'. A's wronging B is not more painful to those who see, hear, or contemplate it than B's being wronged by A.But then that is not evidently true and may even be false: plausibly, the doing of a wrong is more painful to contemplate than the receiving of it. (After all, that's why we judge it to be more shameful!) And thus the refutation fails.
If....[2'] had been the agreed upon definition, the question Socrates would have had to ask would be, 'Which is the more painful to see or hear or contemplate?' hence 'Which is the more painful for those who observe or contemplate the two events?' To that question the answer is, at best, indeterminate. Polus might have argued with some plausibility that most of us would find the former more painful than the latter and, on that ground, that it is "uglier", just as he had maintained at the start: except in rare, abnormally soft-hearted, souls he might have urged, resentment is more easily aroused than pity, more strongly felt and more disturbing to the one who feels it; hence most people would be more pained at the sight or thought of prospering villainy than that of suffering innocence (458).
Vlastos goes on to call the refutation of Polus a 'hollow' victory (459). "Plato himself misjudged the facts which he depicted" (459). (The article is "Was Polus Refuted?", American Journal of Philology, 88:4, 1967, 454-460.)