Did you happen to set up either a second device that can access the vault or created a .csv backup file?
Alternatively, you should have an email with the title "Your backup identity" from the email "noreply@avast.com". There is a link that is provided in this email that you can use to regain access to your vault if you happen to know the ORIGINAL Master Password.
Other than these options, I really don't think there's much that Avast could do to help you retrieve the data. It's encrypted with a password that you created so there would be no way for Avast to decrypt it. If you didn't set up a second device to access the vault, then the passwords were only stored locally on that machine and when you performed a format, all that data went along with it.
Edit: I noticed a post by the moderator of this board that had additional ways to change your Master Password however, this is a post from February of 2017 so the post might not have valid information anymore but...
If you didn't enable synchronization, you can reset the Master Password from Avast UI. Navigate to: Avast! UI > Settings > Passwords. You can cancel the Master Password Dialog and there should be a link that says "Forgot your Master Password?" on the top of the window. You can click that to get rid of the Master Password.
If you did enable synchronization, they would have to reset it, themselves.
Like I said, not sure if this is still valid but worth a try.
