How long does the Christmassy feel (and activities/events) typically continue in Salzburg, Vienna and surrounds? Does it continue past the 25th or is the plug pulled, so to speak, after Christmas day?
Dont know, but if they pull the plug on Christmas day it runs till 1 January in Budapest (2.5 hour train - even for one night)
I would expect the decorations to be up thru New Year’s. The Christmas market in Salzburg is open thru Jan. 1. Some of Vienna’s markets appear to end before Christmas. https://www.visitingvienna.com/sightseeing/christmas-markets/
Christmas markets (Weihnachtsmarkt) in Vienna generally shut down on December 23, some on December 26.
There are a few markets open until January 4 or January 8; they are called New Year's markets (Neujahrsmarkt).
Searching the Internet using the correct terms should give the right information.