Your late night options are a little limited in that neighborhood (which, btw, is more West End than Dupont). I think DC Bread and Brew stays open until the wee hours on weekends. Closer to Dupont proper, Luna Diner has a reasonable vegetarian selection. If you want something a tiny bit more upscale, maybe Afterwords Cafe? I know Bistrot Du Coin is open late, too, but I don't think it has much on the veggie side. If you just want something to grab and take back to your hotel room, there's always Julia's Empanadas.

I don't know of any sushi places around there that are open after 11, though I'm sure there's something I'm missing.