Outdoor dining in Vienna in December?

You don't say how long the trip is. I am very concerned about Covid and am never without my N95 s. I don't take my mask off from the time I enter the shared van to the airport til I am outdoors at my destination- it can be a real marathon. And I'm also vegan and gluten free!
I won't eat indoors at all, anywhere. I chose a hotel for my upcoming visit to Canada with a kitchenette and plan a combo of to-go/take out and easy grocery finds along with random provisions for the plane and snacks from home. Like me, finding options should be easier in big cities like your destinations, if you can't find outdoor dining.
Certainly not everyone would go to these lengths, but it has worked for me- healthy wishes for a wonderful trip!

I remember having a meeting with a friend at 12AM on December 24th, 1999, in front of St. Stephen's main door. I arrived there a little earlier, at 11.55AM and spent those five minutes looking at shop windows. By the time it was noon, I was almost frozen.

Even outdoor heaters typically won't make it comfortable enough for dining - they're typically OK for a drink or two. And they are increasingly frowned upon, or banned outright, due to energy conservation concerns. You might find places with a mostly enclosed, but still outdoors, heated area: not sure that will change much regarding your concerns.
I hate to have to come to this conclusion but ... You might need accommodation with a kitchen.

I'm not speaking from first hand experience in either Prague or Vienna. Been to nothing in the winter, but just dont remember. But I do live in a "neighboring" city with very similar weather and very similar tourism; and yes, we have places with outside dining. It's "cool" out there even with a few heaters but you can find them.

Two other strategies that might work for you are 1) cook at home in your airbnb or have the food delivered. I suspect both cities have Wolt or something similar that brings food from most every restaurant in town. I am home with the flu right now but I just had the best Indian food in town. 2) eat dinner before 7pm and there will be fewer in the restaurant.

Thanks, everyone! I appreciate your kind replies. I suspected that it would be too cold, but I'm glad to have confirmation.

We are spending a week in each city, and are staying in Airbnbs in both. So, we have a kitchen for breakfast and can get carry outs from restaurants for dinner, but during a day of sightseeing, we won't want to waste precious time by going "home" for lunch. And I don't want to miss the Vienna cafe experience entirely. So we'll be as careful as possible and hopefully will stay healthy.

Just wanted to mention that although they are not always the most veggie friendly places, pretty much all the Christmas markets have outdoor eating areas. Granted, these are not sit down places, but might be a good chance to get a snack and/or a warm drink without going inside. At Schönbrunn Christmas market, there are definitely heaters (I was there a week ago) and the market has some of the nicest craft and gift stalls in the city.

If you're willing to travel, I think you need to loosen up a bit on the no indoor dining bit. You're at much higher risk on planes and trains and in airports, that is where I caught the URI I had upon arriving in Vienna.

Anyhow there are lots of cafes with outdoor dining, but here's a look at what it was like a few days ago. No heaters could help you here.

https://www.youtube.com/watch?v=8MJuXyrW1LU

Are you vaccinated against COVID and flu? If yes, you should be pretty safe, even indoors.

Phred. Not this ? https://youtu.be/q7JSwLklDJk?si=qXXZuNu0BZQR4rO0

We've just come back this week from Vienna where there were light snow flurries and it was very, very cold. I didn't see any outdoor dining, and I should imagine that's due at least in part to the fact that it's freezing. The streets are narrow in the centre of the city, and there's no space for patios. I really think you'd be severely limiting your Austria experience if you rule out indoor dining, but obviously self-catering is the main option.

Christmas markets have been correctly mentioned as one outdoor option, but I didn't see a single vegetarian option other than dessert. But a nice spot for a snack and drink.

If you are concerned about indoor dining, there are still options - many restaurants we went to were fairly quiet. In terms of cafes, the ones that are well known had huge queues, but we visited a few that were quiet. The L. Heiner Hofzuckerbacker branch in particular wasn't that busy.