I am sorry to hear you are worried about your dog. I am a veterinarian, and will do my best to help.
Your vet is recommending she be spayed because inguinal hernia is an inherited condition, and any female puppies she has may also have the problem. In general, we recommend not breeding dogs with inheritable disorders, like male dogs with an undescended testicle.
Another reason is that even though the hernia seems resolved, that area could still be weak. If she gets pregnant, as her uterus enlarges it will put pressure on the hernia area from inside, and she could herniate again. I once saw a pregnant dog that this happened to, and a puppy was growing outsider her abdomen, in the hernia. As the puppy got bigger, the situation became life threatening for mother and puppy as the hernia (in this case, a puppy) became a "strangulated hernia", where the pressure was too much. Emergency surgery saved them both.
If this were my own dog, I would spay her, and at the time of spaying I would check the hernia area and if it was weak or there was still a hole in the inguinal area where another hernia could occur, I would fix the hernia at the same time as the spay.
If you really want to breed her, then I would have the hernia area checked to be sure it is not going to herniate again. If there is a chance of it herniating, then that should be fixed before you breed her.
I hope you can see what I am writing in live chat! I am not seeing responses from you.
I will try to connect with you in another format if I don't hear from you soon.