After re-installing Windows 8.1, during the wait for updates of an hour or more, is the computer vulnerable to being hacked (if it's running the Windows Firewall and an Anti-Virus program)?
Short answer - yes it is.
Would I worry about it? No, not really. The chance of it being discovered and exploited within that timeframe is not very high.
However, if you're being targeted continuously or already have infected systems on your network, your chances just skyrocketed.