There are a lot of dimensions to this question, and you've done well to narrow it.
I'm going to assume that you want to balance the pragmatic results of your actions with not increasing the overall demand for meat in the world.
In this case, and for this set of facts, it's not unreasonable on the face of it to eat the meat- the pragmatic effect would not be to increase the demand of meat.
However, if you factor in the behavior of the waiter and/or restaurant that served you meat that you didn't order, you may be able to influence someone that serves many meals a day, some of which go to vegans, to not make the mistake as frequently going forward. This could be as simple as politely sending the meal back accompanied by an explanation that you didn't order it and don't eat meat. You would have a potential chance to offset the waste of a single meat entree with several or many over time.
Even if you choose not to do that, many vegans might choose to offer that meal to someone they are dining with who does eat meat, or taking it to go and giving it to someone else, thereby not wasting it but also not eating it themselves.