How about trying this after you've gone through the vinegar/penetrating oil stage,
Drill a hole sideways through both sides of what's left of the stem as close as you can to the fork
Thread a nut or threaded race down onto the fork threads (make sure you use something with really good clean, long threads)
Put a long bolt or piece of steel rod through the hole in the stem, so that it sticks well out on both sides
Wind the nut/threaded race up the steerer till it hits the bolt (use a spacer between them if need be to keep the nut/threaded race well engaged with the fork threads)
Give the nut/threaded race a bit of a twist with a big spanner to apply some tension to the bolt, pulling the stem out of the fork
(assuming you've got the stem wedge out) If the stem doesn't come out, turn the fork upside-down and using a mallet and a long rod down the steerer give it a whack (if the wedge is still in, this will just jam it in tighter)
Repeat 5&6 with more penetrating oil until it comes out. Keep an eye on the fork threads to check you're not mangling them.
How about trying this after you've gone through the vinegar/penetrating oil stage,
Drill a hole sideways through both sides of what's left of the stem as close as you can to the fork
Thread a nut or threaded race down onto the fork threads (make sure you use something with really good clean, long threads)
Put a long bolt or piece of steel rod through the hole in the stem, so that it sticks well out on both sides
Wind the nut/threaded race up the steerer till it hits the bolt (use a spacer between them if need be to keep the nut/threaded race well engaged with the fork threads)
Give the nut/threaded race a bit of a twist with a big spanner to apply some tension to the bolt, pulling the stem out of the fork
(assuming you've got the stem wedge out) If the stem doesn't come out, turn the fork upside-down and using a mallet and a long rod down the steerer give it a whack (if the wedge is still in, this will just jam it in tighter)
Repeat 5&6 with more penetrating oil until it comes out. Keep an eye on the fork threads to check you're not mangling them.
Good luck!