"does a multivitamin trigger insulin and will effect my fast? "

I guess it depends on the amount taken.

If you take very tiny amount (as you are water fasting, taking too much vitamins MAY have negative effect ) , you may not get affected.

But there is certainly amount threshold, after which you will get affected somehow.

WS.

Partial eating is never good while on a water fast.

Let body do its work. Taking Vitamins is based on fears. I like Dr. Mercola, ask him, he will tell you.