cruise?

Those are river cruises on Danube. Look at AMAWaterways, Viking, etc

Maybe what you heard is that sometimes there is difficulty due to water levels. Viking goes quite a bit further than Vienna, like Germany. It's about an 8 days cruise.

As far as I know there is no regular shuttle service between Vienna and Budapest, unlike e.g. the daily cruises to Bratislava.

Going from Vienna to Budapest by ship is normally part of a longer Danube cruise.

This is a 4 day Vienna-Budapest-Vienna cruise (operated three times in October):
https://www.ddsg-blue-danube.at/kreuzfahrten/ms-primadonna/4-tage-wien-budapest-wien/
(This page is available in German, only.)

As downstream of Vienna the great Hungarian plains start, the scenery along the river is not pretty impressing.

Yes, wmt1 got it right. This is not about water levels or someone asking about a major river cruise on Viking. There are no short cruises between Vienna and Budapest, mainly because it’s not very interesting and the train is so much faster.

Emily, the OP had stated that there were cruises and now they were no longer available. Yes, that stretch is boring. But if the OP felt there were some and now there's not, OP could be confused about the many news reports about cruises on the Rhine and Danube and the issues they were having with water levels.

Now, your response pointing out that there were never cruises because it is a boring part of the river, is likely accurate but I was responding to the OP and addressing his/her impressions.

Michael,

Avalon Waterways offers 4 day cruises from Budapest to Vienna or the reverse.

According to this article in USA Today, the only provider of that ferry service closed in 2017.