A pair of vice grips won't help. You've got a galvanic corrosion problem where your iron bolt is stuck in an alloy hole. The only way to cure this is to heat the bolt and get it red hot. THEN, a pair of vice grips will be able to remove it.
If you don't have access to welding equipment AND the car is drivable, take it the nearest shop and ask them to do it. Go there armed with FOUR replacement bolts and any required washers and gaskets, so that you can drive away again. It should set you back 20$ for somebody else to take care of it. Also, be sure to lubricate the bolts slightly prior to reassembly.
If you have made friends at a junk yard or other equally wonderful, slimely place, they most definitely can help.
If it holds water with 3 bolts now, it won't do so for long.