So it's important enough for Spock to try and save Vulcan and Earth but everyone that died because of Nomad (four billion inhabitants of the Malurian system), The Doomsday machine (at least two star systems and probably more not to mention the threat to the Orion system), the space ameba (billions of inhabitants of Gamma 7A system and the Intrepid) should just hope that Kirk just happens to be in the right place at the right time?
Don't forget, in regards to the number of surviving Vulcans, both Spocks were still of the opinion that the race was in danger of going extinct. If there were millions of survivors on hundreds of planets you'd think that one of them would have thought of that by then, not to mention the Federation. Would it really be the best thing for the survivors to be dumped on a new colony planet as opposed to one of the older, more established colonies? "Sorry you just lost your planet and most of your people. We're going to stick you over here on this uninhabited planet so you can start over from scratch." Even for a Vulcan that's pretty cold. It does make more sense if it's intended to be a new homeworld though.