I agree you can’t be everything to everyone. And that doing it proper is key since vegetarians and vegans tend to give more scrutiny to what they eat, in general.
I’ve made more money selling tofu dogs at an event in Seattle than I have anywhere. So, you don’t have to be in Berkeley for veggie dogs to go over well if you are doing them right.
The thread owner is located in Sacramento. I don’t know the city well, but I know its in California and that its a large city. You tend to have lots of niche groups represented in large cities. Enough to sell more than a pack of alternative dogs a day, if you try.
If you don’t want any part of it Djdawg, that is OK. Someone else will do it. Just like I don’t worry about putting traditional “meat” hot dogs, pulled pork sandwich, etc, on the street, because its being done at just about every corner market and in between.
 
The title of the thread is “I want to sell…”. So, are they an idiot for wanting to do something like this? I don’t think so. I think they are using their head. But maybe we both have our bias.
 

Closeminded?  Whatever….a veggie/tofu dog makes about as much sense as a fat free Twinkie.  It’s just idiotic.  If by closeminded you mean I will try and do a few things very well….and not try to cater to every Tom, Dick and Harry, then so be it…I’m closeminded.  You can’t be everything to everyone…and it’s obvious the hassle of trying to server a vegan dog in the proper way is way more work than it’s worth and may even cost you business.  If I had a cart in San Francisco or Berkeley Ca, then it would probably be perfect. 

Yea, definitely look into cross contamination issues if you are offering both meat and veg. It may not be worth it.

And obviously, the success of this alternative will depend on your community. Case in point, the closemindedness of people like djdawg as presented in his reply. Do some research with your customers or perspective customers, then make an educated decision. Good luck!