I just wanted to report that I've solved my problem with one of my Vegas being quite spastic, like less than half hashrate of its sibling. My Windows key was deactivated for a while now, and I finally contacted the supports and they helped me reactivating my Windows copy. I got an update, and after restarting both cards are running at the same speed:

https://i.imgur.com/Sxk4zwu.png

Edit: Um, it seems I started celebrating too early. My 4th miner report shows it dropped to around 1100h/s again...