There is probably some artificial color or preservative in the multivitamin that you are reacting negatively to. A vitamin/mineral supplement can not cause acne.
Long story short - stop taking that particular multi vitamin and switch brands. The board sponsor n2bm.com makes a really good multi vitamin called ruthless 3D. Check that out and maybe give that a try.
Unless there is an additive in there causing a reaction, then it has to be something else. Just stop taking it and see what happens. Try switching to Ruthless 3D multivitamin. I have been using it for years and have never had an issue. N2BM sells it and it was designed for athletes.