I've not heard of only taking vitamins in first trimester. 
I've always took them all the way through and then continued whilst breastfeeding switching to the breast feeding variety.
Definitely speak to your care provider before stopping anything especially Metformin - I'm guessing if your a type 2 diabetic then your under consultant care so ask them before making any medication changes.
My understanding is Metformin is often prescribed in pregnancy so no reason to worry / suddenly stop before seeking advice.