How would I go about recreating beef/hamburger taste/umami, strictly vegan?
Cooks Illustrated has an ultimate veggie burger recipe that you can adapt. Their key to umami is cremini mushrooms.  I've made that recipe and it was well received. Of course, no one mistook them for real hamburgers, but the patties tasted quite good. 


MSG (monosodium glutamate) is to umami flavour what sugar is to sweet flavour. So if you're pro-msg, you can try that, too.  And pleanty reasons not to ;)
I recommend trying shitake mushrooms as well.
Also try up to 5% wheat germ/flour which should help with the caramelization and related flavours. 
grated vidalia onions in the patties also help (caramelization and umami).