It wasn't that Ethan wouldn't have pegged Vasily for a father it just hadn't ever factored into his equations when he worked with or trained alongside the other man.
"Presumably from before you got taken by the Regiment?" Because the Regiment was no place to raise a child. Ethan didn't know what had happened to his family after he'd been arrested, he presumed that they'd just gotten on with life and were either missing him or pretending like he didn't exist.
Honestly he didn't know what he would do if for any reason the Regiment ceased to exist.