Is it stated on screen that Spock was told in advance that Nero was going to destroy Vulcan?Some points:
Spock was there at least long enough to somehow find a cave and a piece of wood, (?) and create fire. Maybe we can chalk this up to Nero being gracious in a weird villainous way. Sort of like a jailer giving a cigarette to an inmate. But if that isn't the case, then Spock's guilty of some pretty extreme negligence for not trekking to the outpost that he knew was there and trying to warn somebody.