Jake,
Are the vinyl graphics for sails made from the same material used on hulls?
I doubt it. The vinyl being applied to the hulls would be quite a rigid type, the vinyl applied to the sails would be a more flexible type probaly more often used on PVC banners and such. This is so you can roll your sail up and the vinyl wont crack/delaminate.
If I wanted something on my sails i'd screenprint it on myself, or me or a mate would airbrush it on. I've had a look at my sail and its dacron, I have inks that would last as long as the sail would likely last.
Because of the nature of the abuse a sail gets (bashed by wind, salt water, rolled up after use) I'd imagine a vinyl label/graphic to last no longer than 2 years applied to a sail.
Though I did get some dacron with an adhesive backer from a sail-loft down the road to patch a couple of what looks like cigarette burns in my sail. I noticed that my sail # must be produced from the same stuff, and it has no signs of wear and tear.
And I imagine it is possible someone has a digital printer that can print onto this substrate (Dacron with adhesive backer) which can be applied to a sail that should last years!. I think my boat was made in 1983, I imagine the sail is original as well, so that sail # has been there 20 years using the adhesive backed Dacron.