..... are you vulnerable to someone brute forcing the password encryption on the watching only wallet.dat file (if that is a thing)
It isn't a thing. WO wallets cannot currently be encrypted, and they only carry public data.
wallet.dat is Bitcoin Core's wallet format, which Armory does not use nor depend upon. Loosing Core's wallet.dat is an issue, as those carry private keys.
Armory's .watching_only.wallet files only carry public data. Compromising this file will leak your privacy. Any attacker can also choose to swap addresses in that file for his, so that you would serve his addresses instead of yours to receive payment.
Armory checks the consistency of all public data in wallets at start to prevent this sort of attack vectors (and data corruption in general, never know when a disk/stick of ram might fail).