You can't do this with Ledger AFAICT, it uses HD wallets under the hood.

They're BIP39 passphrases, so there isn't a way to convert the key you have to a suitable passphrase.

You could write your own vanitygen to derive hierarchical keys where the derived path for Ledger (they probably follow a BIP for this) happens to match the prefix you use, and load that onto the ledger wallet.

I'm still not sure why you would want to do this either - only the first derived key will match your vanity prefix, and unless you start using the HD chain it's a wasted exercise IMO!

Key re-use is bad, mkay.