You could stay home and take a vacation in Chicago!
If you're looking for something further afield:
All the northeastern big cities are pretty vegan friendly (not sure about Philly..never been there). DC is fantastic, and a nice vacation spot. Actually, I suspect most big cities are decent. In the past few years, I've been in geez, 30 large cities in the States and have never struggled. Even in Oklahoma, I could manage as a then mostly-vegan. Traverse City (MI) is actually quite nice as well. It might not be a spot for a "big" vacation, but it's a nice beach getaway.
On this side of the sea, I've had good luck most everywhere in England, in Brussels, and in Barcelona (which gets a bad rap veg-wise, but it was fab for me). Paris was challenging, though that was in part a language barrier. Norway was not naturally vegan-friendly, but with some hunting, IS and I have eaten well, and even had a fancy-schmancy meal at a restaurant on our last visit.