I'd like to find some sort of "spoke wrench" which actually clamps onto the nipple (not the spoke proper), so that nipples that are slightly rounded-over (or, like aluminum ones, have a propensity to round-over) can be adjusted without further damaging the nipple.
The Park SW-10 is exactly what you are looking for. It can really save the day. If possible, use it (and a healthy dose of light lubricant) the moment that you notice any stripping to completely remove and replace the damaged nipple. I've seen very old versions of identical design, and wouldn't be surprised to learn if it dated back to penny-farthing days.
I have several nipple spanners. None of them are clamping as per the other answer, but some are better than others at not rounding off the corners.
There's a park spanner on the right which has flat jaws. Its pretty good at rounding off a nipple if not firmly engaged, or if there are not quite perfect tolerances.
The middle one is another park took that has a pair of lips - you have to put it around the spoke and then slide it down on top of the nipple. This works better than the flat sided one.
On the left is some random chinese thing that I picked up a long time ago. Notice the lips are much stouter and longer. It also has 4 different sizes of fitment for the different widths of nipple.
As long as there's pressure outward from the hub, this style is good and is unlikely to round a nipple.
4 inch vice grips will probably not preserve the nipple, but IMHO if you can't turn it with the correct size spoke wrench, the nipple should be replaced. These wrenchs are pretty cheap, if you ground the jaws flat it might help preserve the spoke nipple. They will definitely let you get enough grip that you're either going to get the spoke loose or break the spoke if the threads are totally locked.