There's a hundred threads about vinyl removal in the archives. Sounds like you have a pretty large project ahead , and if you have to ask for tips, you've probably never tackled one like this before.
What kind of shape is the vehicle paint in? Have a straight up conversation with your client about damage that may result from this removal, and let him know what might happen.
You'll find several techniques work well, some low, some on the roof. You'll find out what works best as you go.