Try to use Liquid Wrench, PB Blaster or some other type of penetrating oil.
With vice grips you may be able to get a good enough grip on what's left of the bolt to remove it.
You can file down opposing edges of a bolt or nut so that there are 2 flat edges and then try vice grips again. This may be enough to give the vice grips enough of a bite.
If you have a propane torch you can apply heat to the bolt to help loosen it. Be careful using an open flame around an engine bay or any other location where there could be flammable material.
Use a bolt out set, these tend to work best if you have enough room to hammer them onto the stripped bolt.
Use a hack saw, air cut off tool or a Dremel to cut a grove into the bolt head and then use a straight blade screw driver to remove it.
Cut a slot in the bolt head with a hack saw, cut off tool or Dremel. Once you have the slot cut use a straight blade in a impact driver to help remove the bolt. Be careful not to use this on aluminum or any other materials that are easily damaged.
A drill out set should only be used when the head of the bolt has been snapped off and the rest of bolt is recessed in a hole. Be sure to run the drill in reverse when drilling the hole as the bits in drill out sets are meant to run in reverse. Also, make sure to run the drill out very slowly as they are prone to breaking. Make sure to drill the hole in the direct center of the bolt. A center punch can help to make sure you have the exact center of the bolt.
Drill the bolt out with the proper size drill bit that will fit the Heli-Coil that you need to put in.