I forgot that dragon roll is a real thing for a second, and thought that the joke was implying eating dragon is vegan.

Also, doesn’t a dragon roll usually have shrimp in it?

In my experience it’s eel that’s the ‘dragon’. Although one place around here has fucking chicken in it.
The dragon rolls I’ve seen considered of shrimp tempura and avocado topped with unagi. The eel is the dragon, while the shrimp provides a tail.