It’s not possible to sex a bunny that young, so she could be either male or female – you’ll have to find that out when the baby is older.
It’s really important that you find a rabbit savvy vet – based on what he told you (feed veggies), it’s likely that this vet isn’t bunny savvy because you’re right in saying that the baby is too young to be fed veggies. From what I’ve noticed of my bun, his poops are always darker when he eats too much veggies or drinks too much water, though I’m not sure if that’s the general cause for dark poops.
If the bunny is below 2 months old, then yes it’s too young to be separated from the mother. Could you upload a picture of the baby? A rabbit savvy vet would be able to gauge the age best, but people on this forum may be experienced enough to roughly tell whether it’s a very young bunny.
Young bunnies should be eating alfalfa based pellets, but I’m not sure if bunnies as young as <8 weeks should be eating any pellets at all, so I'll leave that for the others to advise.