Just an update: At VIE I flew out of an F gate, and at F05 there is an customs counter. Used the phone there to call a customs agent whom eventually came and stamped my forms. Even though I was at the airport super early with lots of time to spare customs didn't want to show up till 5 mins before boarding time, which ended up actually being 10 mins after boarding started. But got it all done and got my VAT refund