We made the same transition last october. When we first got the VG, I was blown away by the speed and quality. Now, only 4-5 months later, it is constantly going down. The cut calibration is awful, and I can't keep the print quality consistent. Now I have a print head completely dropped out and waiting for replacement only 4 months in. I put alot of miles on the SP and never had this much trouble. Also ran a VS-540i for years with no issues. I assume your using Versaworks Dual as it comes with the VG, and I have found that it processes colors in .pdf better than it does for .eps. Try sending as a .pdf to see if it helps at all. Good Luck.
Not to derail the thread but I figured I'd give my input here. The VGs need a crop mark filter installed or else crop mark detection is a pain. Apparently it was figured out and the SG machines come with it already installed, but the VGs do not. Once we had that installed, cutting and cut calibration has been a breeze. We have had some issues with cutter downforce but those were largely cleared up with the newest firmware (2.20). Fingers crossed we don't lose a head so early, do you know what caused it? Our local tech has been pretty interested in helping with our machine though because there's only a few installed in our area and she wants to learn more about it.
Back on topic
I can't say from color vibrancy though, out of the box the VG has been a bit brighter and bolder than our XC 540 was especially the greens. The PDF trick was also recommended to us by the installation tech, but our EPS output has been great so we haven't used it. We're running it in dual CMYK mode, although it's my understanding that the ink setups don't affect gamut just gradient smoothness so I wouldn't think that would have a dulling effect like you're describing.
Is there a checkbox somewhere that's overriding your selections? The output should change noticably between say Pre-Press US and MAX Impact. Also, the ink system is different in this machine from you're old machine so you may need too choose new colors if you don't have the ability to profile to match the old ones. There's probably not gonna be a setting that just outputs the same or similar colors.
My opinion is that the VG was shipped too early and we are sort of beta testing it. A lot of the issues we've had have been relatively small. There are some engineering flaws though, but this thread is about color so I won't go into it.
Biggest problem I'm facing right now is an issue with versaworks dual, but I think it has to do with a faulty install that I haven't had time to work out.