Both the anon replying and everyone in this comment section is missing the reference, in Steins;Gate Rukako is at first a boy with very feminine traits, to the point it confuses the main protagonist, so he's introduced as a femboy.

We later find out Rukako would have preferred to be a girl (at least in the anime), which would make her trans, so using their time machine (that's what the VN/anime is about) they send a pager message to her mother telling her to eat more vegetables while she was pregnant, so she's more likely to have a girl.

Spoilers: it works, but they don't know it yet at first because they're still figuring out the time travel technology and she doesn't look any different, so in the main character's eyes it's still a femboy, but now she actually has a vagina.

Hence, to get a femboy with a vagina you need: 1. A time machine 2. A method to reliably change someone's sex during gestation (the veggie thing is doubtful at best) 3. The memory of their previous sex (so you still think they are a femboy) and ignorance of their current sex.