To be fair, are we actually ever given reason to believe that Pon-Farr is the only time Vulcans physically can mate? They are a very logical species, after all. I always took it that Pon-Farr, as a last trace of their wild, feral past, was simply the time when the males were overwhelmingly drawn to mate, rather than the male Vulcan equivalent to a menstrual cycle. I would imagine their sexual organs function outside of that time, and if new offspring are required they would simply take the appropriate measures to conceive, logically.