How does vanity HDM (multi-sig) wallet generation works?

If you only know cosigner xpubs then where does the mnemonic come from?

If I feed this tool N xpubs, I will only get a N+1 of N+1 multi-sig setup, in which only the first address meets the vanity rules - did I get what you meant?

Also, are the pubkeys in a multi-sig setup sorted by default?

I think you may make this more clear in README, otherwise a noob like me will feel quite confused.