Scoring 20+ PPG has nothing to do with it, Trey Murphy is a much more efficient scorer overall (better shooter from the line and from 3, more range, less reliant on the mid-range). For reference, Murphy's career eFG% is 61.2 vs Vassell's 55.4%. He's probably overrated on defense, but he's not worse than Vassell, not by a long shot. More importantly, as a true wing / forward with serious length (6'8" with a 7' wingspan), he's a much better fit than Vassell for the current roster.
His contract is similar to Vassell's (16% for the cap for Murphy vs 17.5 %to 13% for Vassell since it's descending) but what makes it more palatable is that it'd be a better use of the money, since it's less duplicative. Unless Vassell consistently shoots well above average from 3 on high volume, his skill set isn't worth his salary slot, whereas Murphy's probably is if you see him as a legit starter on a good team. All in all, like I said, I think he's become overrated recently but that is a different discussion from being better than what we have, if we can turn Vassell into Murphy for a reasonable price I think it's an improvement.
PS: Sorry to hear my takes are getting worse, I'll take the weekend in penance to reflect.