Bigvan, sometimes bite marks haven't so much worn away the vulcanite as they have compressed it. Sometimes a little heat will raise them up.
To expand on this, this is my method.
If the stem is in decent shape (like the one pictured):
I'll clean up the surface with micromesh, but this is just to get rid of any gunk that may have formed a layer around the button area.
I then heat up a needle (ok, so I use a metal scribe) to red hot. Put a drop of water (some people say distilled, I say foo on them, tap is fine) in the offensive dent, and apply the tip of the needle. If the tooth-mark is a compression dent, this will raise it back to "flat" (you will still need to fine-tune with micromesh or sandpaper).
This method also works for raising dings in bowls and wooden furniture.
If the stem is chewed up something fierce (VULCANITE ONLY!!!):
I simply bring a pot of water to the verge of boiling, dip the button end of the stem for a few seconds, then remove and allow to cool. Repeat as required. It also behooves you to keep a pipe-cleaner in the airway if you use this method, to counter any collapse.
The biggest downside to this method is that it causes mass oxidization, and will require some serious elbow-grease or grinder time to bring back to black (or a bleach bath), but it does work wonders on destroyed buttons.
Bite-throughs: Every so often, I'll bite the bullet and bid on a lot of stems on Ebay. While some of these will be cleaned up to replace questionable stems on estates, the majority end up in a box. If I have to repair a bite-though, I'll grab one of these stems, and start sanding until I get a decent pile of (vulcanite, derlyn, or lucite) dust. I'll then mix this with a quick set epoxy, and fill the offending hole. Hit the area with sand-paper to bring it back to flat, and you should have an almost invisible repair.
BONUS - Hallmarks (makers marks):
I will often simply allow the original paint to dissolve, then re-paint with a fine brush and White-Out. If the design is more intricate and I don't want to try and recreate it, or uses more than one colour/metallic colours, I'll put a dab of Vasoline over it before introducing any of the liquid restoration methods. The same goes for aluminum parts (tenons, etc). I use a detail sander and dental pick to get into the crannies in these areas.
