I took Russian through high school, so I feel somewhat qualified to answer this accurately.
Vasya can be used for both
Vasiliy and
Vasilisa, like
Zhenya can be used for both
Yevgeniy and
Yevgeniya, and
Sasha for
Aleksandr and
Aleksandra.
In Russia, they don’t really use nn as a stand-alone fn. So if you met a Russian
Vasya (whether male or female) it’s safe to assume that’s only their nn.
In a non-Russian setting where it wouldn’t matter as much (though would perhaps be a bit humorous if anyone knew better) you could certainly use
Vasya as a fn, but I would personally prefer it as a nickname for
Vasilisa (since were talking about girls here).
“He who takes offense when it is not intended is a fool, and he who takes offense when it is intended is an even greater fool.”