Spock went back in time to an ice age, and fell in love with a woman named Zarabeth, as part of his biological regression. Violence among Vulcan's was common at that period, but in biology the purpose of violence of that sort is about reproduction and selection.
Spock himself is proof that Vulcan's and humans can have viable offspring.
Did Spock introduce Vulcan DNA to the Sarpeidon gene pool? Would this qualify as a violation of the prime directive? Given how amazingly technologically advanced the civilization was, could that level of advancement be correlated with introduction of Vulcan DNA into the gene pool?
Update: In the novel, Spock and Zarabeth had a son. http://memory-alpha.wikia.com/wiki/Yesterday%27s_Son