I suggest you try applying some heat. Hopefully you haven't rounded all the corners off and use a 6 point socket.
I'm with Gordon. I would try a heat gun. Heat around the plug, not directly on it. You want to have the housing expand more than the plug.
I don't know which plug you are working on, drain or fill. I always remove fill first as I don't want to disable the bike by draining a housing I can't refill. Vise Grips will grab almost anything, but you need to meet three criteria.
1. They must be real Vise Grips, not knock-off locking pliers from Harbor Freight.
2. They must be in good shape with reasonably sharp teeth, not well-worn rounded teeth.
3. The pliers must be TIGHTLY applied. I mean with the screw turned in so far that it takes both hands to barely get the handle to lock.
Then trash the plug and buy a new one. Check the housing for thread damage and clean up with a tap if necessary. Use a good quality calibrated torque wrench (again, not Harbor Freight) to install the plugs. Do not exceed the BMW torque spec!
Personally, I find BMW's torque values on the high side for the fastener size. I now routinely reduce the torque by 10% on reusable fasteners. I use the book values for one-time use fasteners such as clutch cover bolts, flywheel nut, etc., structural fasteners such as swingarm pivot bolts, and any bolt or nut that is setting a bearing preload. Otherwise, 90% of the book value is sufficient to keep the fastener in place, yet makes it less likely to damage threads or make the fastener impossible to remove after a few years. However, this is just my judgement and you will have to make your own.