Strangely enough, I'm pretty sure Vampire Weekend was the craziest mosh pit I was involved with that weekend. I'm even going to include the awful time I was crushed against the fence during Kid Cudi. The Vampire Weekend pit was actually kinda fun.
Probably on the Yeti, to be honest. I would imagine a set similar to J. Mascis' this year.
Jeff Mangum solo seems to be a much bigger deal than J Mascis solo, though. And Neutral Milk Hotel (arguably) has a much bigger legacy than Dinosaur Jr. Also, I can't imagine Mangum ever appearing in the light of day...