Weeeeell we could just be vague about the dad, have him be absent/AWOL and just not be mentioned... I can see them dancing around it or it not coming up at all. At Billy's current canon point, all he knows is that Wanda was married and had kids; teen!Vision was cut off before he could explain how it worked in YA, and Billy knows she basically wished herself pregnant, so there's no reason she couldn't do the same thing in Luceti, just sans the husband.

If you'd prefer there be a dad, that'd be fine, but I think this would be easiest?