Very sound advice Lew,

Always, Always, talk to your printer or service provider prior to converting or working in CYMK.

CYMK is totally device dependent and there is no one solution.  The default photoshop North American Prepress setting are designed to meet the SWOP standard. It not an Ideal setting for a specific output device.

What is SWOP,  Specification Web Offset Publications.  It's a standard color target for large run web offset presses. The kind producing catalogs, magizine, and large run color.   It a common agreed upon standard that should result in reasonable color consistacy across presses, vendors and print runs.  As a result,  it not an ideal conversion for the best color on a specific device, press or workflow.  It's a comprise that the major agreed upon.

About how do they get such vivid color on vinyl?  It's hard to tell without knowing the equipment used. But several large format inkjets are not just CYMK.  Many have  6 or more color primary inks or special saturated inks. They typically  use special drivers or "RIPs"  to convert your RGB  or CYMK to produce optimun results at a greater color range then CYMK on its own.

The substrate or paper has an large effect on color reproduction.  Traditional CYMK inks are transparent.  Light passes through the pigment, reflects off the paper below so a highly reflective clay coat paper will produce better results then say recycled newsprint. Which is why Epson and Hp offer "special photo paper" its designed to among other things, to reflect more light back to the eye repoducing more saturated colors.

Hope this helps.

J. Stuart J.