That being said.......the owner or pilot in charge of the aircraft will have to get YOUR installation signed-off by a licensed A & P. (Airframe & Powerplant). A log entry must be made by the A & P verifying the install to be airworthy. After that is done you are not responsible...he is. So, BEFORE you do the job I would have a little meet & greet with the A & P so you know what he will sign for and what areas not to cover up.
Not so simple...
Technically, since the vinyl does not have a part number from the manufacturer of the aircraft, it could be viewed as an Alteration/Modification. As an alteration or modification, you'd need FAA- Form 337. You would need to submit the approved data, which would be signed off by an IA, and approved by the FSDO. The FAA may even do a field inspection, if they are not familiar with the IA submitting the paperwork. Depending on the FSDO you are working with, it would either be difficult or impossible to get an approval. (BTW, tail number decals have part numbers). This is why I initally posted to contact the flight standards district office. They are the ones interpreting the rules, and they are ultimatly the ones to determine airworthiness and the legitimacy of a repair or modification (installation). You'd have no problem painting the plane with an ap's signoff, but installing vinyl could be open to interpretation of the rules.
At the owners risk, once an aircraft is modified and it is found to not have the proper paperwork, it is essentially unairworthy, thus rendering the aircraft un-insured, since the first line in the inflight portion of the insurance doc's require the plane to be maintained in airworthy condition. You could have trouble with liability, should a mishap occur.
And before someone chimes in about Cirrus (and others) airplanes having Vinyl Decals on them, they are part of the type certificate, in that the plane was certified with the vinyl decals. The side markings have part numbers.