Why did Vance change his legal name to his grandmother's last name as an adult?

Because he was raised by her and felt more of a connection to her?

It's unusual, but not unheard of. I personally know people who've taken on their grandmother's name, or mother's maiden name, or some other family name that isn't as directly or traditionally connected.

"Vance" was his mother's name, and her parents' names. JD Vance grew up in the Vance household. His birthname was his father's, who, as far as I can tell, left the picture when JD was very young.

For the record, all of that doesn't mean I trust Vance. That was never something I said. I'm always suspicious of politicians, and it's sadly usually warranted. My point is, I'm loving what they're doing so far, and I'm focusing on the good, which is unpreceded and amazing. If and when the bad comes, I'll condemn Trump/Vance/whoever then. Not before.

Is Vance a snake? Possible, sure! This admin is kicking ridiculous amounts of ass, though, and hasn't plunged a knife into my back yet.