There is no vitamin or anything that compares to breastmilk - it cannot be reproduced because it contains many live cells including white blood cells. Multi-vitmains have around 20 ingredients, breastmilk has several hundred and it changes based on requirement / age etc. So a vitamin cannot match that

I say feed as long as both mother and baby are happy and willing. You may even change your mind as you go or things may occur which change things for you. When I fell pregnant with Elijah, after a little while my milk disappeared and Marisa weaned.