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.
Understood. I just have all these private keys I've gathered over time written down that I want to use. I REALLY want to use them. I even tried coding my own daemon to recieve transactions on a 1Decoded address and forward it to me, even put it through a mixer. Didn't work, though.
Another question, can the Trezor import wallets to do this?
(It can sign messages so IIRC it's not HD.)