Keep in mind vDIMM depends entirely on what type of RAM you're using, TCCD chips with brainpower PCB loves voltage and you can generally max out your vDIMM with 'em (unless of course you're using a lanparty with 4v options). The nice thing about TCCD chips is raising the voltage will often allow you to tighten up your timings.
Same being said for generic value RAM, chances are you try and pump 3v through them they'll end up frying, I suggest investing some cash in some new RAM since your current stuff has horrible latency. I've seen some Twinmos floating around here than can handle high vDIMM and has really tight timings for 100 bucks or so.
As Nubius mentioned, it's crucial to run prim95 and memtest86 every time you increase your RAM speed, if it errors or crashes that's a sign you should loosen up your timings.
Intel C2D E6320 / AMD Athlon X2 3800+
Gigabyte 965P DS3 / DFI nF4 Ultra-D
2GB OCZ Gold PC2-6400 / 2GB OCZ Gold PC4000
eVGA 8800GTS 320MB / eVGA 6800GS 256MB
150GB Raptor / 74GB Raptor
2x500GB / 320GB
OCZ GameXStreme 850w / OCZ StealthXStream 600w