Should I give my toddler vitamins?

Maybe. Although most children can get all vitamins and nutrients they need from a well balanced diet, there are some times when a multivitamin can be helpful. Vit d and calcium are often lacking, especially in children who can't (or won't) drink milk. Iron can also be deficient in children who do not eat red meat.
Yes, if off formulas. The general recommendation is that toddlers should be given extra vitamin d unless they are already taking in 400 units a day from dairy in their diet. Kids can be picky eaters who don't eat a balanced diet every week, so giving a children's multivitamin each day is an easy way to be sure they are getting adequate amounts of a variety of vitamins. Toddler formula has the vitamins added already.
Maybe. A natural drop in feeding happens as growth slows from a 14lb/10in/yr in 1st yr to a 5lb/4inch/yr in 2nd. They also drive you crazy with their food selections. I expect todlers to feed well about 4-5 days a month, grazing & picky for the rest. Vitamins are ok if you want but most do well without. I like liquid vits at that age, not the candy vits. Todlers may find & ingest adult look alike meds.