I have seen some impressive poison ivy in my days of wandering the mountain paths behind our home in Roanoke, Va.
Some of the old poison ivy vines on the slopes of the mountains there can be three or four inches in diameter which is pretty scary if you haven't battled poison ivy of this size previously.
The technique that I have always used, and it seems to work on any poison ivy that has used another tree to gain altitude, is to first spray enough Brush B' Gone or similar product to open up a little space on the trunk of the tree.
Poison ivy vines are pretty easy to identify. They are hairy and grow into the tree. Usually Virginia Creeper and other vines don't grow into the tree.
Once I have killed enough of the poison ivy to get close to the tree, I take an ax and sever the vines. I try to cut out one to two inches of the vine. There are usually multiple vines.
Typically this will force the poison ivy vine to come up in other spots closer to ground level where you can attack with more Brush B' Gone.
Check back on a regular basis for new explosions of poison ivy vines which will come up from the roots. Sometimes on old growth poison ivy, this might take more than one growing season.
Keep using the Brush B' Gone until you have whipped the poison ivy. You can win.
It is really good to get rid of these high growing vines as they often produce lots of berries which have seeds which the birds spread all over the place.
I have been on a crusade to get rid of poison ivy before it gets me. I hate it, and there is nothing better than taking out an old vine which has been torturing people for years.