Use VanitySeach for faster speed and for possible GPU support (depending on your GPU) instead.
Does VanitySearch also support 12 or 24 seeds? If necessary, it also works without would only be nice because of the compatibility with other wallets.
No, it doesn't; just like Vanitygen.
Or is there possibly a possibility to convert a private key afterwards to a seed? I think I read something that you convert it into bites and then the bites into words.
You read it right but it's not the way you think it is.
If you're aiming to use the private key of a vanity address and convert it into a seed phrase in hope to restore the same vanity address, it wont work.
That's because the addresses derived from a seed phrase aren't based from the initial entropy but from the mnemonic itself (
the 24 words) which is used to derive a "
seed".
Not to mention that the vanity address is derived from the private key in a specific way, not the same way as deriving private key and addresses from a seed.