Priority Pass let's you visit 4 lounges in VIE (Air Lounge, Jet Lounge, and the 2 Sky Lounges) but in your case the most viable option is the Air Lounge.

It is located right after passport control in Terminal 1 from where BA is flying (as a matter of fact BA uses it for it C pax too).

As said it is right after passport control but before security as the security check is done at the gate right before boarding.