Complete list of co-factors (magnesium being the most important one):

magnesium zinc vitamin K2 boron a tiny amount of vitamin A

For more info... http://www.vitamindcouncil.org/newsletter/more-vitamin-d-que...

Sorry, I don't have an existing set of references for this, but here are a couple things I found:

From http://www.drjessicaseaton.com/Chiropractic_in_West_Los_Ange...

"Increased levels of vitamin D require increased levels of vitamin K, especially of vitamin K2 (menaquinone). Without enough vitamin K2 there is an increased risk of hardening of the arteries and other soft tissues."

From http://en.wikipedia.org/wiki/Vitamin_K

"...high intakes of vitamin D but low intakes of vitamin K were suggested to pose an increased risk of hip fracture."

http://www.lef.org/magazine/mag2010/sep2010_Brittle-Bones-Ha...

Here is the summary: "The remarkable discovery that blood vessel cells can transform into bone-forming cells confirmed the little-known link between atherosclerosis and osteoporosis. At the core of this connection is insufficient D and K intake. These vital nutrients operate in synergy to optimize bone mineralization and prevent calcium deposits in vascular tissue. Low vitamin D is linked with arterial disease and bone loss, while vitamin K stimulates bone formation and modifies specific proteins (Gla) that help protect against arterial calcification.

Vitamin D taken in higher doses (5,000-10,000 IU/day) has become popular over the last two years based on findings showing that this potency is required to achieve optimal blood levels (over 50 ng/mL of 25-hydroxyvitamin D). Relatively few supplement takers, however, understand the critical need for aging humans to also take a daily vitamin K supplement. Fortunately, Life Extension members were informed in 1999 about the critical need of including vitamin K in their supplement regimen."