I am analyzing vitamin-D3 by simple method.Hope it is helpful for u.
Column: kromasil C18 250*4.6mm 5u
Mobile Phase : 100% Methanol
Flow Rate : 1ml/min
Detector: uv at 254nm
Diluent : Mobile phase

Try this method if you r using pure form of D3. If it is in matrix form heat the sample with mobile phase for 5-10min.