A good Google search term is "vitamin D biosensor"
As of spring 2010 it has been prototyped and patented
It will probably be too expensive initially to have in your home.
But I can see having it in a pharmacy - where is would be 100X more cost-effective
Many large grocery stores have pharmacies.
And many of those pharmacies have chairs for blood pressure testing
I can imagine going to the grocery store in 2012 and getting a vitamin D blood test for < $5 in <2 minutes
There may be legal restrictions against it, however, so you might instead have to go to a medical facility, fire stations etc.