Such footage exists for Brosnan and Craig, but I've never seen any for Dalton's Bond. Could this IMDb entry suggest it exists somewhere? https://m.imdb.com/title/tt5631108/
There are plenty of photos and videos from the Vienna press conference (you can see a few clips of it on the making of documentary on the DVD as I remember) but I don't think it was really 'announcing Dalton as Bond' in the same way that Craig got unveiled and Brosnan had his Robinson Crusoe press event at the Regent Hotel- the Vienna one was after they'd been shooting the film for a few weeks. I think it was the first chance the press got to talk to Dalton though as he'd been cast so close to shooting.
One thing I do think is quite fun is that apparently Aston Martin boss Victor Gaunlett drove his own car all the way to Vienna for the press conference 😊
Ah.... The Eighties😍