Welcome back theoverclocker9!
My recommendations would be to start with the core voltage. Avoid changing the VRAM until you get a stable VCore. You have some very good options with the Radeon Wattman software. I would start at the State7 (Max) and bring it down 25 and State 3-6 down 10. Run Firestrike or your preferred GPU benchmarking suite and test for stability. Once instability is detected bring up your numbers until you are fully stable again. Like Johan stated, this will be a trial and error on your part, just like overclocking. Once you have a stable undervolt on the VCore, save those settings and bring them back up a little more for VRam tweaking. Follow the same procedure. Once a stable undervolt is achieved with the VRam plug in the VCore undervolt numbers with the VRam undervolt and retest for stability. You'll want this to be your longest stress test and it will be the final undervolt.