How can I get more umami in a veggie burger?
Umami comes from natural glutamates. Two excellent vegan sources of umami are tomato paste and dried shiitake mushrooms (rehydrate then mince). Fresh shiitake aren't nearly so high in glutamates. They are available very inexpensively at Asian groceries.

If you want vegetarian, but not vegan and can find a rennet-free parmesan-style cheese, they are also very high in glutamates.