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Vegan cheeses, as already mentioned, vary considerably in melting characteristics.

All the various ones I have experience with do not melt at similar temperatures to cow cheese, or to the same extent at the same time/temperature profile anyway. The Violife is the best (at melting) we have found yet, as well; usually the "cheddar-like shreds" (even on pizza.) It can be a bit like the way chocolate melts, in that it will melt while holding its shape, but can be smeared around physically as melted.

In the context of Not Superheated pizza (thus, not pizza at all for the folks that won't cook one below 900°F/482°C) as done in a normal oven to serve people who are happy enough to call it pizza anyway at 450°F (232°C), I have found considerable benefit to turning the broiler element on (supposedly "low" rather than "high" for whatever that might be worth) in the already hot oven for a brief period - 2-3 minutes as the pizza bakes and I watch carefully though the window. This is not a step I need to take for dairy cheese. 

I would expect the broiler trick to help with cheeseburgers as well, since it applies heat preferentially to the cheese on top.