I certainly can commiserate on that one. I worked on a poison ivy vine for about 5 or 6 years which was growing up between some azaleas near the house. You have to get the root - that's the key. If there is any of it left after your treatments and its still alive, the poison ivy will definitely come back again and again. Have you tried taking a shovel to it and digging up the root? On older plants the root can be quite large. The one I worked on, the main root had a radius of 1 1/2" or so. It was quite long as well. I just kept chipping, shoveling, and pulling until it was gone. I also painted on some killer directly onto any root I couldn't pull up. Persistence was the key.