If you can extract yourself from the situation or somehow get a letter from someone who is not a jerk, by all means, consider doing it.
It really is a disappointment to hear these stories of nasty and mean vets.
You can either leave, tell the person off and leave, or bow your head, swallow it and do your best to earn the respect (or whatever positive spin you wish to say) of this doctor.
Trust is a big thing when it comes LORs... I think if there's any doubt in your mind whether this vet can write you a positive letter - you need to flat-out ask and hear the answer. It would be terrible to trust this vet as an evaluator and be knifed in the back on a LOR.