Pardon the delayed reply...was expecting an email notification when somebody replied to this post but didn't get one! Thanks to both of you for your replies.
I've just been going into my local Avast install, clicking settings, and then using the Exclusions drop-down. I wasn't aware you had to do this in the Cloud Console instead of on the local machine...that seems to render the local Exclusions settings totally useless unless I'm misunderstanding. Seems like it'd be logical to remove it from the general options if this is the case.
Anyway, I've set up a new group settings template in the cloud console and applied it to my system so I guess we'll see if the settings stick now.
Thanks very much for your help!
P.S. Sorry, noticed you asked for more details on the alert...when I run 'vagrant up' it says vagrant.exe is suspicious and asks if I want to create an exception, but even if I do it's still halted the operation so I have to kill the process and restart. It also does this with ruby.exe upon 'vagrant up'. FYI, in case you're not aware Vagrant is an engine used for quickly spinning up virtual machines...pretty widely used by web developers. I'm running version 1.9.8 on Windows 10.