To further on both previous comments-
WD40 is pretty poor as a penetrating oil. It was never actually designed for that. As such, you may notice that last year they started offering a line of 'specialist' products that I believe include a penetrating oil however they aren't all that common yet. Liquid Wrench is another brand of penetrating oil that has shown to be one of the better products in testing. Kroil and PB Blaster are others, though I think Liquid Wrench probably is a bit better than either of them as well.
If you do go with heat, you may want to flush all the fluid out of the system since you'll likely cook it. At least do a good bleed to flush the caliper.
Lastly, I wouldn't worry too much about destroying the bleeder with vice grips. Yeah, they'll likely mangle it pretty badly, but you're going to replace it anyway, right? And even if they totally round it off, a bleeder would be relatively easy to use a bolt extractor on since it's already hollow down the middle. BTW, if you have the oportunity/need to use an extractor very often, look into something other than the EZ-Out brand stuff. The tool truck guys all offer things that work much better than the typical parts store things, but of course they charge accordingly.
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