How hard to break a cipher if it has a different key for each word?
A Vigenère cypher can be unbreakable if, and only if:


the key is random,
the key is at least as long as the message being encrypted
the key is used only once.


In that case a Vigenère cypher is mathematically equivalent to a One Time Pad, and the same mathematical proof of unbreakability applies.

You proposal is closer to being unbreakable than standard Vigenère, but it is not an OTP either.  Hence your proposal is not unbreakable, but is more difficult to break than a standard Vigenère.