But even using the lax biological rules of Star Trek, I can see no way that two beings with differing blood chemistry could breed without help. The mother provides nutrients for the Foetus - it's her blood and her oxygen that keeps the little fella going. I don't see how a human can keep a Vulcan foetus alive even assuming Vulcan sperm could be genetically engineered to penetrate the wall and fertilise a human ovum if they have completely different blood.
Maybe that's not how Vulcan women keep their foetuses alive of course. In nature, hybrids can vary based on whether you have, for example, a tiger father and lion mother or a lion father and tiger mother. Even so, I would have thought that Spock could not have grown in amanda's womb, in spite of the scene from NuTrek (cut I believe) showing his natural birth.