Water soluble vitamins will be supplied as a soluble powder. Based on the strength, calculate the amount required (i.e. mg/litre) and gently introduce into milk with agitation using a standard mixer or high shear mixer for 5 minutes.
In the case of W/O type emulsions, by keeping the high shear mixer running the emulsion stays W/O. I am not sure if the emulsion separates when you stop the mixer.
If you experience sedimentation on standing for a long period, you have to experiment with W/O ratio and drop the temperature as low as possible without changing the characteristics of the emulsion.