I haven’t tried them all, but I agree, veggie dogs need to be grilled not steamed. Then you can hot hold them in either with a false bottom with some water underneath, or in griled onions.
And a word to the wise, if you want to really cater to the vegetarian or vegan crowd, even if to offer them an option, I would research the term a little so you don’t caught trying to offer them fish, or soup with chicken stock. Vegetarian means NO meat (including chicken and fish, for those who don’t consider that meat). Vegan means no dairy (cheese, etc), egg, meat, honey, refined sugar (bone char used to process).