Vulcanite and Ebonite are the same thing, really. They are both vulcanized rubber.
There's a lot more than just acrylic. I have made stems out of acrylic, polyester, juma and a few others. Some are harder to work and sand than others.
With a few exceptions you really can stop sanding at 600. Especially if you wet sand from 320 up. It really depends on how much you want to sand and how you want to sand.
As far as buffing, I use Tim West's green compound, followed by red tripoli and then white tripoli.
A note on buffing compound. DO NOT be tempted to think of the grit number given to buffing compound as the same thing as the grit number given to sand paper. If you sand a stem to 1000 and then buff with green you ARE NOT taking a step backwards. Sandpaper and buffing compounds work differently and cannot be compared in any way. If you have a buffer, and different buffing wheels for each compound, go ahead and use them in the order you would on anything else you are buffing. (I like the Beall system).
I hope I have helped.