Hi Dan
Each time you teecut you take a thin layer of paint off. PSA is known for using very little top coat.
The trouble with a previously lettered van is the the paint under the vinyl is still new as it has been protected from the elements, where the paint not covered has been damaged by the elements.
The best you can do is make sure all the sticky adhesive and any ridges from the old lettering has been removed. Once the new signage has been put on nobody, except you, will notice it.
I have a 13 year old Suzuki Carry that was owned for the 1st four years of its life by a painter. 9 years later and 2 teecuts, I can still see his name and details in the right light and angle, but its hardly visible. It hasn't be re sign written.
Save your money and put it toward your new signage.