What to use to get latex paint off of oil-based?

The person living in my house before me painted many of the doors and walls with latex paint, which was over oil-based paint. They didn't use a primer to make the latex stick better. Now the latex is peeling off in some spots, and I am working to get it all off. What is a good product or something to aid in the removal of this? I used vinegar on other paints and that seemed to work well.