I love getting customer supplied files with CMYK values instead of pantones. Most of the time it looks pretty close... But theres always that rare occasion where it comes out slightly purple... so you go and check the pantone... find a CMYK value, then have no idea if it's what they want or not.
so you can eyeball it... use your color calibrated monitor to get it pretty close... send it to the customer only to find out their monitor is shit, and they wanted it to be a completely different color. The one that shows up on "Their" monitor. It's always real estate people that do that to us...
But generally... yes, if you have the vinyl in hand.. buy a color muse. Its $50.... scan the color, get the LAB value... put it into onyx or flexi and hit print. If you have good profiles it'll generally print pretty close... Then a bit of tweaking and you should be there.
We just had a vehicle we printed on our solvent printer come back for a whole panel redo from a crash. It was a weird pinkish color... I thought I wasgoing to have a hell of a time matching the color. We didn't have a sample of the pink in our records for some reason... And I wasn't going to cut a piece off to match it with our xrite... So we got a color muse for $50. scanned it in.. hit print... and it was bang on. I thought for something so cheap it'd be off...especially after I got the box and it said "91% accurate". but it worked great. a tool everyone should have.