1.) Not create a black hole, as that would doom Romulus too. Instead, change the gravitational pull of the star (increase it) just enough for it to balance out the forces trying to blow it apart or keep it from collapsing.
This, of course makes no sense by any understanding of Physics.
2.) Spock knew that the star would collapse into a black hole and doom Romulus. But that threat would be less immediate than the SuperNova, buying Romulus and friends time to evacuate Romulus and resettle the population in a system less inhospitable.
If Nero had every intent of turning planet Vulva into a black hole, he would want to drill to the center to ensure the proper formation of collapse, which would be the ONLY part of the plot which would make any sense in any understanding of physics that we have. Because this exotic Red Matter that is a goopy fluid they can contain in a glass jar having the ability to form a dinky little speck of rock into a rare and powerful black hole most certainly defies even the most ignorant understanding of physics.
The crashing of the ship containing a large amount of Red Matter into Neros ship creating a black hole is pretty bad. Adding to it the Enterprise detonating a series of Warp Cores (Seriously, it launched a buttload of them at the thing) and riding the wave away from the gravity well of a Black hole that it could not escape while warping space without it being reduced to subatomic particles is far worse.
The reboot movie was Character Development and Special Effects only. It was not a reboot, IMHO, as it allows little room for a bunch of follow up films. But as far as the science in it, it cannot be taken with a grain of salt, even.
Rather, you need a few hundred kilos of Atropine.