He wouldn't save Vulcan by going back. He would just be creating a NEW timeline. Prime Vulcan would still be around. NuVulcan would be dead and NuNuVulcan would be saved.
I'm assuming these are the new time travel rules since Prime Vulcan is around.
edit: Now granted, Spock may not be aware of these new rules...how could anyone? Unless there are some differences he may have discovered.