Vanya is a nickname for Ivan in Russia, also used in Belarus and Ukraine. But in Bulgaria I believe it's a girls name, short for Ivana. But as a Ukrainian who grew up around Russian speakers I could not imagine a female Vanya.
In Russia diminutives end in -ya a lot regardless of gender. Tonya, while not labelled as on the site, is also a boy's name in the Eastern Slavic countries. There are plenty of people named Anton that are called Tonya here.