Air-conditioned room a must?

Depends on how tolerant you are of heat.

Seeing that the 'averages' are hi 80 low 64, I know I would want a/c

To be sure, you can expect the weather day in and day out in Vienna in the summer to be hot, ie, in the 80s F , sometimes oppressively so when you're out and about. It was like when I was there last summer as it is whenever I'm there, which is always in the summer. If you need the AC, then absolutely take such a room.

Having AC is not a necessity for me. I don't need it.

We were in Vienna last June. Our room did have a/c but we never used it. In August, maybe we would have. We found that a/c wasn’t a given in every hotel that we looked at. It took a little work to find a room with a/c in our moderate price range.

It may well be hot enough that I, at least, would need a/c. Only dire emergency would lead me to accept a room in central Europe without a/c in August. It will mean avoiding the very least expensive lodging options. If you are on a budget and think you might like to play odds, lcheck out the actual, historical, day-by-day weather statistics available on the website timeanddate.com. They go back at least ten years; I'd look at the most recent five, because weather varies a lot from year to year. I've just done that, and I see 90-degree days every August.

Vienna weather -- August 2023

Use the pull-down box at the right, just above the graph, to change the month and year displayed.

I was in Salzburg several years ago the first week of August. Temp hit 90 for a couple of days. I was in a hotel without air conditioning. I survived 🙂, but it would have been cool to have a/c.

Will there be street noice in the evening or at night near your hotel? One of the advantages of AC is closing your windows to keep out the street noise. Even if the nieghborhood looks quiet (e.g. no trains or trams), there could be a late night bar or club under your window.

For me, ac is absolutely necessary in July and August. If I don’t get ac I’m sure to get an attic room with small windows directly across from a bar that’s noisy late into the night! A hot, stuffy, and loud room doesn’t make for a good night sleep.