Your're a wind up merchant LHD. There is no way you would have got past check in or got a boarding card with the wrong ticket.
Yes if you used the self service then why check in for another flight with a wrong ticket, what are you trying to prove.
Even if you did use the SS, all the BA flights I work on or been on you would not get passed the gate with the wrong boarding card, its all automated, and you would not get past the cabin crew on boarding without the correct boarding card.
Once checked in you do not have to show your ticket again.
Confirm you didn't have the correct boarding card for the VIE and you may sound credible