yellow urine means that your renal tubuals in your kidney are retaining water because you are dehydrated. drink more water, dont drink any cafeine, and avoid salty foods.
and yes after a vitamin your pee will be yellow if:
1 your body doesnt need the vitamins/minerals
2 you are putting too many vitamins into your system at once (ie your vit has TOO MUCH stuff in it and you should get a smaller vit and take it a few times a day)

hope this helps
if you are running any cycle with orals to protect yourself get some milk thistle and cranberry extract.

MM