Veganism is an ethical position, not a diet. It to reduce the harm to animals we cause as much as feasible and possible. A veggie burger cooked need to a beef one doesn't harm any animal nor does it add demand for animal products. Ethically there's nothing really wrong with it.

Same with the last, I'm vegan because I can live fine while lowering the harm I do to animals, but meat and dairy products do taste good.