achieve brighter reds when printing to silver metallic vinyl

uncertain which metallic vinyl you're using but they typically have a grey-tint to them and the bright-point only comes at the right viewing angle.
a white base would give you the ability to print the "brightest red" possible, but if that's not an option then 3M's Gloss Dragon Fire Red is pretty damn bright, and metallic. I'd be hard pressed to find a brighter red-metallic wrap vinyl via other brands, I stopped looking after my 3M/Avery/Hexis colorbooks.

got pics for the thread to haggle other options?
maybe post what material you were using as well and the CMYK value that your RIP sent to your printer, just to rule-out anything you can possibly tweak via your current printer/media set.

 

You're putting metallic lettering on a truck. Why print it? Just buy red metallic vinyl and cut it. Stick it on. Done. Not wrap vinyl, just regular cast vinyl. From your favorite manufacturer. They all make it. Avery even makes an ultrametallic with bigger flakes.

A 24"x10yd roll is going to be around $100. For small amounts Fellers will sell cut yardage.

 

nah, not here to rehash your approved design.... OP was how to make it work.
i wouldn't be printing a bit of it, that looks like that could have easily been cut out of either metallic or reflective stock colored vinyl.... print&lam just added to your cost.

design-wise, if we were to touch on that..... AE's eagle-logo would have looked nice on the door panel, with the flag&text flying out back from it.

 

print a Pantone colour chart on the silver metallic and see if there is a brighter one

 

I'd personally run some test colors on a clear and stick the swatches to a bit of silver to save material. I've also noticed differing effects when printing on chrome vs printing on clear applied to chrome . Is this a 'mirror finish' metallic or a "metallic sparkle" type vinyl?
You may also try generating a perf pattern on your print, so you maintain the color red and yet have some of the silver show through to perhaps fool the eye into thinking it's brighter, but it still won't compete with the unprinted stripes. At the same time this will make the red appear pinker from a distance I think.
Just spit balling thoughts here....

 

Oh boy.