I'm not sure how vise-grips would help. My flare wrench has a good grip on the nut already and I've tried hitting it with a hammer.
I would consider the repair kit but I think it requires a few inches of straight line where it can be cut and spliced; my line has a bend right next to the filter. It looks just like the image that Spike posted above, and it's the left side of that image that I can't get undone.
In that image, the small nut is around the line and should turn freely of the line; the large nut is welded to the filter body. Unfortunately, you can't just crank the hell out of one nut; doing so will twist the whole assembly and break the line. You have to hold one nut and turn the other (or turn them against eachother).
I'm using proper flare wrenches:
which provide a good grip that won't slip or round the nut. I've managed to brace it against the frame in such a way that I can whack it with a hammer to make it turn and it didn't seem to get anywhere.