Hmm.... I'm thinking this thing is not going to budge... I soaked it in WD40. I tried Sheldon's bolt method, and all I did was round the corners on the nut. 15" Crescent wrench has mostly rounded the corners of the cup. I tried the vice again, and I think it's too mangled now to have a chance at getting a good grip on it.

Ah well... I do have a Barracuda A2E frame that aside from having shorter chainstays, should also work for what I want. Thanks for the ideas on trying fix this.