Check out this video by Dr. Mercola. I know it is long, but there is some great information about which blood test are accurate to optimize Vitamin D levels. Great stuff. You can scroll down and see the written summary underneath the videos.
The two most important pieces of information:
1) Use D3 for oral supplimentation
2)The proper test is the LabCorp 25(OH)D level.
Click on the link below for more detailed information about Vitamin D levels and which tests to ask your doctor for.