There's a lot of chat about copper, and measurably, copper is a better conductor than most of the other common materials (SS, brass, etc).
However, you'll find a great deal more of the VD in any given mod in its contact connectivity than in its material. The threads, positive post, and ESPECIALLY the button.
Have a good look at pbusardo's alternate method of measuring conductivity, and gaze in wonder at the el cheapo telescopic "arachnid" mod that competes pretty well with copper and brass mods.
It's in the button.
The more pieces in the circuit, the more places that the electricity has to jump from one solid conductor to the next, the more VD.
That's why a stainless SMPL will measure out to pretty darn close to a copper interior Stingray X.
Ultimately, if you REALLY wanted to do ultra low VD right you'd design a hybrid atty to thread onto your mod tube. This makes the positive center pin of the atty 1 solid piece with the positive contact for the batt, and gives only one thread contact with the tube. If you make the atty in such a way (see Tobh, for example) that the negative posts are integral and one piece with the body, there's another removed contact point. Then, a well designed, consistent and low VD button. The basic design elements of Stingray, nemesis, etc. is a tried and true design that could use a little refinement, but it's the same basic button design that reappears in the best.
Make the whole thing out of fine .999 silver. Then have the threads and contact points gold plated.
You probably wouldn't be able to measure any VD without seriously pricey research grade equipment.
I've even toyed with the idea of doing a kickstarter to make this a reality. There's a machine shop down the road from me who charges fairly, does one offs, small runs, and prototyping, and is generally a reasonable place to do business. Silver is kinda expensive, but not prohibitively so. Etc.