MalevolentSin666 Yep, the vaults are a double-edged sword. You have a place to dump excess funds, so your household cash has room to grow above the 10 million cap. But when bill time comes, you get high bills because whatever you add to the vault is added to the value of the lot rather than seen as cash. I've never seen bills be more than the cap though. I don't know of any game cheat to reduce the bills owed amount nor do I know for certain of a mod that do it either. There might be an option in MCCC but idk as I've never needed it.
Hope this helps.