I found this link helpful in explaining out of gamut stuff:
http://mrprepress.wordpress.com/200...amut-color-space-no-one-will-hear-you-scream/
Basically you will have trouble with a few very specific colors, and the closer to those colors you are, the worse it will be. Some people figure rgb can create anything, but there are cmyk colors that are out of the RGB gamut, not just vice versa.
The only solution I like is to print a color chart on whatever printer is going to be doing the final work, and have the customer physically come in and pick a color off of that chart. I do that even if it means I am printing 5 little 1" color boxes on the 600 pound roland. You can either do one big color chart for all jobs, or a chart specific to this current job (which I think a customer would appreciate.. like if they want a certain green I'll print 10 variations of that green). Once they pick a color, you're covered, they will like the results, and you don't need to fool with printing test strips.
I would advise strongly against any method where they try to match based on what's on their monitor, or your monitor, or a printout that didn't come from the correct printer. Even a pantone book won't look the same as brand X glossy printable vinyl and will require trial and error on your part. If they want to specify a certain set of CMYK or RGB values, that gives you a starting point, but nothing beats an ok from a real person.
