That's not immediately clear. Depending on whether you think it's second or fourth declension, and masculine or neuter, the various possibilities are virua, vira, virūs, and viri. My best guess is that the "correct" usage was one of the more exotic varients (virua or vira), but the word was so rare that many people didn't learn the nuances, and instead adopted it to the more common patterns. Similar to how "begs the question" is often used incorrectly, and that ends up becoming an accepted usage.

However, the plural could not have been "virii."

More than either of you two probably want to know about the subject.