LND specific answer
It seems to generate ECDSA public keys using a BIP43 like keychain system commonly used for wallets.
So ultimately it is derived from a BIP43 like key derivation mechanism based on the comments. Like what you would see in a wallet.
I don't think there would be anything stopping you from modifying this and just hardcoding a custom vanity key.
Or somehow generating a path/keyindex that derives your vanity key.
However, most node public keys are commonly displayed in base16 so you don't have a ton of options other than making a vanity like DEADBEEF1234567890...
Given the limitation, I'm not sure if that would be worth doing. Probably better to just define yourself through an alias.