I went to UTSA all last year, so I can say a bit about SA.
How old are you first off? Who you mingle with may definitely depend on your age, as well as what part of the neighborhood you live in. If you live closer to UTSA you may run into a couple Conservatives, and a couple more Christians, but they're usually timid and those that aren't are stupid and easy to shut down.
Now if you live near the Trinity area, you're going to have fun. Nearly everyone there is liberal, antitheist, and vegetarian. They have tons of fascinating speakers visit their campus all the time, and most are open to the public.
Unfortunately I can't say much about the 30+ crowd, being 18, and I apologize for that.
As for being a vegetarian, it is slim pickings. It's not entirely impossible, definitely easier than Houston, but not as easy as Austin. I would suggest greek food at Papouli's, Green which you must already know about, The Cove, Pei Wei, and a few others.
And finally, I think it will be a no on the bike. Most of SA is based around I-10 so it's definitely not safe to bike, this does depend on where you live though.