I was having the issues too, i had to RMA my VX4000 to cure the problem.
The situation i got from speaking with all the OCZ guys and their communication with DFI is this:
It is an IC issue, the VX is designed to take high voltages and can make certain mobo/cpu combos have a tough time detecting the memory.
The temp cure:
Get your rig to boot off the one "GOOD STICK" in slot 2 then set the voltage to 3.2 in bios and fsb to 200mhz and on VX 2-3-3-8 timings (manually select spd)
Then pop in the other stick, it should post...from here you can overclock away but DO NOT power down the system, resetting is fine but hard reboot will cause the mem to not be detected.
After this....go call OCZ and get an RMA going, my new stick are clocking higher and i can turn my computer on and off at will (what a luxury!!)
The ram isnt defective, its just incompatible in certain situations.
Good luck guys, dont worry OCZ has awesome RMA support you can read my post on the OCZ forums
here:
http://www.bleedinedge.com/forum/showthread.php?t=9573
Forums on the BleedinEdge - Is this a candidate for RMA?