I have fed vita plus to all of my horses forever, even while feeding omolene 200 which is a completely fortified feed and they say that you do not have to feed any vit or min supplement with it. Most vit and min that are in supplements will actually be (for lack of a better word) peed from their system if they do not need it. The only thing you have to be careful about is things like selenium, vitamin a and d. These are absorbed into the blood stream and can not be peed out as easily if overdosed.
As far as switching feeds, when you start to run low on cadence, decide which feed you are going to start him on and make it a choice you are going to stick with for a while until he gets older. Start by adding a handful or two in place of the cadence the first day. The gradually, over at least seven days (I do 2 weeks) work him up to where he eats no more candence and all of his new feed. In the middle of the 7 or 14 days you should have his feed 1/2 cadence and 1/2 new feed. this way you lessen the risk of the new feed upsetting him and causing a colic episode.
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"