When do I use my account master password versus the vault master password? It seems like sometimes I need one, sometimes the other?
It sounds like you may have a Primary (standalone) vault on your device, as well as a 1Password account, does that sound accurate?
If that's the case, then 1Password for Windows will unlock using your Primary vault password, but your 1Password account on my.1Password.com will unlock using your account password.
Let me know if that makes sense, and if that's the case, we can also help you migrate any remaining standalone data to a 1Password account.