I watched the episode featuring Trip and T'Pol's ill-fated Elizabeth. It seems to imply that Vulcans and humans can't reproduce without medical intervention, as apparently happened when Spock was conceived more than a hundred years later. But wasn't there an alternate timeline in Star Trek: The Next Generation featuring Sela, a Romulan-human hybrid who was conceived when the Romulan father assaulted Tasha Yar? I doubt that the Romulan went to the trouble of paying a geneticist to produce a viable human-Romulan embryo when the mother was his slave/captive. Sela would tend to suggest that Romulans and humans (and probably humans and Vulcans) can have kids the old fashioned way in the Star Trek universe.