I am trying to replace a vanity and sink in the bathroom. I can't get the valves to turn- they won't shut off. If I really force is with pliers the heads seem to slip. I am afraid of stripping them.

When a supply shut off valve like that sticks and will not turn you are best off to replace the valve. If you have a 3/8 X 5/8 compression valve it is easy to replace , especially if you use the same brand to replace it with. Just turn off the water to the house and unscrew the 5/8 nut and the 3/8 nut and remove the old valve, now you can install the new valve without having to replace the 5/8 nut. You also can use the old 3/8 nut and the old supply unless you are planning on using a new and different kind of supply.

Another way to do this if you have the same brand of supply stop is to turn off the water and unscrew the packing nut that holds the stem in, after that is loose unscrew the stem and replace it with the stem from the new cutoff that you bought. Sometimes this way will not work and you have to resort to the first way.