The Secor restaurant does have a vegan dog on its menu. We ordered it with vegetarian chili on top. The dogs were tasty, though basically the sausage tasted like a tofurky-style brat, which you can find in nearly any grocery store. We applaud Packo's for having a clearly-labeled vegan option though. You can take your out-of-town guests here and have something to eat yourself.
Note that currently only the Secor restaurant has the vegan option (let us know if you find it elsewhere). Also, the vegetarian chili is vegan and that is available at all of the Packos restaurants. Finally, there is a great pickle and pepper bar with lots of fun and tasty options to try.