If you can get an impact driver on the head
of the bolt, then gently tighten and then loosen. Also hit the head of the bolt with a hammer. Keep repeating until the vibration has broken rust free and then the bolt will come out. Heat is also a good choice as the heat will expand the metal. I like to heat it very hot and then let it cool for several cycles. The expansion and contraction breaks the grip of the rust on the threads. If you think you will break off the bolt, make sure you have straight access will a drill and bit before you put that much pressure on the bolt. If you break off the bolt, drill out the interior
of the bolt using slow RPM's with medium pressure on the bit. If you break off the bit, you are screwed. Use larger drill bits until it is just about the size of the interior
of the thread on the bolt. You will need a sample of the bolt to determine the final size of the drill bit. Use a tap and clean the old bolt threads out of the seat. If your bit walks on the bolt and you over size the hole, you can opt for drilling and tapping a new hole slightly larger or perhaps use a helicoil to replace the threads.
If this spooks you, then find someone who has done this before to hold your hand. The first time is always the worst. PS only put vicegrips on a bolt as a last resort.