I tried to free a stuck stem once by putting the stem in the vise, putting the front wheel on and then twisting the wheel. Managed to snap off a fork drop out!

Now my approach is to put the fork crown in the vise (with padding), dribble a 50/50 mixture of automatic transmission fluid and acetone in there (from the top and also flip it over and dribble some in via the bottom of the fork crown) and let it soak for 24 hours. Then use the handlebars as leverage. If it doesn't move at first, another round of ATF/acetone, and try again.