Some walls in mobile homes are put in with long staples. Once you get an edge loose it should be easy to put it off the rest of the way. That's the good news. The bad is, sometimes the molding trim pieces break up when you take them off even if you are careful doing it. Take a piece of the wall with you to a big lumber company and they will help you find the right thickness you need, it may not be the same material the orginal was though. Also take the measurements with you and they will probably cut it for you. Buy paneling nails to nail it in.

You're right, you're most likely going to need to replace that. You don't need to wrack your brain trying to find a matching sheet of vinyl wallboard, I matched the thickness of my wallboard to 3/8'' drywall, which you can get anywhere. It's not complicated to do, but it helps to have a 2nd person and do a little research first about hanging drywall so you're not completely in the dark. Taking the wall panel off will be easy though. You can use screws or drywall nails to attach the new piece, I recommend screws. Good luck!