If this article is to be believed, in the 1880's it was already being threatened on it home turf by Bavarian competition:
"A BEER EXHIBITION.
(FROM THE DAILY TELEGRAPH CORRESPONDENT.)
Vienna, Friday Night.
Next April a Beer Exhibition will take place in Vienna. The committee includes the principal brewers of Austria-Hungary and several of the Viennese restaurateurs who organised the Cookery Exhibition held here in the spring. The idea is not improbably due to the competition of Bavarian beer, which has within the last few months largely increased in Vienna. Three or four vast establishments for the sale of Bavarian beer have lately been opened and are doing well. This is a serious matter for native brewers, who until now never realised the possibility that Pilsner and Lager might some day be supplanted by Bavarian competition. The fact is that even in Vienna if you want good native beer you must know where to get it. When of inferior quality it produces a malady very common in this country, where it is called catarrh of the stomach. The Bavarian ale that has hitherto made its appearance in Vienna is much lighter than that drunk in Munich and is, in the opinion of connoisseurs, a fair rival of the world-renowned Vienna Beer."
Freeman's Journal - Monday 29 December 1884, page 7.
Odd that they call it Bavarian Ale, eh? Mention of it being lighter than the stuff sold in Munich has me wondering about the colour. Though I thought the first pale Munich Pale Lager was only brewed in the 1890's.
Of course, there's one thing that really did for Vienna Lager: WW I. Vienna went from being the capital of one of the largest countries in Europe to a provincial backwater. No surprise then that its Lager also lost its lustre.