First things first, mastodons were a giant animal related to present-day elephants and woolly mammoths. The name literally translates as "breast tooth" because anatomist Georges Cuvier thought its molars looked like they had nipples.
Image credit: Mary Gagnon
Although mastodons lived as little as 12,000 years ago, there are very few samples of their soft tissue, and absolutely none of their reproductive system. This means, ultimately, we don't know exactly what mastodon vaginas were like.