Mael Seedvault use 10 words for generate the password but the words are predifined.
It's robust like a very long password ?
If someone extract all words, it's secure against bruteforcing ?
A SeedVault key is indeed made up of a sequence of words, with the word list being known.
But a passphrase is made up of a sequence of characters, with the character list being known!
What matters is the length of the sequence, the number of options in each position of the sequence, and whether the items in the sequence are correlated in a predictable way or not.
Often when people pick a password that is easy to remember, it is easy to remember because the items later in the sequence are predictable from the earlier items (e.g., if a password is "crumpe" followed by one letter, that letter is very likely to be exactly 't', so the extra letter is not providing substantial entropy).
SeedVault using a fixed word list is no more risky than passphrases using a fixed alphabet.