It seems evident that your doc wants to establish diagnostic evidence of whether you have anaemia or not. By far the most common causes of anaemia are an insufficiency of Iron, or B12 and/or Folate, which is why they want you to have a blood test to establish your natural baselines.
FBC - Full Blood Count. Primarily to test for your Haemoglobin level. If lower than the established normal range, this is technically anaemia. Secondary tests would be the morphology of the red blood cells, which are sometimes effected by the type of anaemia.Useful information can sometimes also be gained from the White Cell Count and Platelets.
Vitamin Levels - I assume they mean to test for Vitamins B12 and Folate (Vitamin B9) , either or both of which can be indicators for certain types of anaemia.Like Sqad, I am a little puzzled they want to test for this now - I would have expected them to perform the FBC test first, and only if that showed any abnormalities would you then go on to order the other, more specialised tests.It may be that you exhibit symptoms associated with Vitamin B complex deficiency, or you fall into one of the patient categories most often associated with such deficiencies.
If you have already started taking supplements, then obviously the effect of these supplements may distort the laboratory results by artificially elevating your iron or B12/Folate, which is why they wish you to stop taking the supplements, then to book in for a blood test.
The time it takes to clear such supplements from your system is likely to be measured in hours / days, so if you were to book a blood test for 1 week from seeing your doc, that ought to be sufficient.