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.)
HD wallets can sign messages too. The address that are in the HD wallets still have a corresponding private key to it.
Trezor, AFAIK can't do that either. Most wallets disable this feature since it wouldn't allow the user to utilise the change address since all changes would be sent back to the address. Why do you want to use hardware wallets with vanity addresses? Hardware wallets are meant for storage and security, vanity addresses would most likely have their security compromised slightly since most of them would be exposed to the internet or generated on a computer that has been exposed to the internet.