First, line up your Peeps against the glass, one on top of the other. Yes, they are smashed against the glass a bit like prisoners riding into town on a bus. It’s OK. They are used to it.
Second, in the middle of the big vase, place a smaller vase and fill it with water. Put tulips in this vase.
Third, fill in any gaps with fake grass. This will help hold up the tulip stems and it will make the Peeps feel at home. They are bunnies, after all.