There are many reasons why Vegas Pro can crash. The two most common are 1). Your PC hardware, and 2). The media you are using. Maybe if you give some information about those two features someone would be able to help you.

I haven't had a single crash since updated to VP21B300. I use Vegas daily!

It can happen during any action during editing. Or just randomly out of nowhere. It's become non-functional really. What to do?

Where did you buy the vegas ? If on the official site, you can just install the previous build! 

Download here
https://www.vegascreativesoftware.info/us/forum/faq-where-can-i-download-vegas-pro-and-other-vegas-software--104782/

I'm more unlucky, I bought vegas from Steam and it's impossible to download another version.

When you ask a question, try to answer trivial questions right away. What do you use for shooting, what vegas settings  

But is it impossible for you to download b 208 from here?

https://www.vegascreativesoftware.info/us/forum/update-vegas-pro-21-build-208--143825/

Or can it not be installed, since you do not have a serial from steam? (what i do not know).

If you're on Steam I don't think it's possible to get another version and you can't install the one from Magix. Hopefully the current one is fixed soon for those users.

@Wolfgang S.  From previous posts on the forum over the years, Steam Vegas versions (and other Steam apps/programs) have their own registration/activation processes which are significantly different from MAGIX/Vegas sourced downloads. Steam doesn't rely on serial numbers but rather a registration process that relies on the user always being connected to Steam's online 'My Account' website/system in order to use Steam Vegas at the time - and the Steam sourced download is unique to Steam and may not include all of MAGIX Vegas' features such as the ability to activate on two computers at the same time. Steam is a universe unrelated to the procedures that may apply to those same products if purchased from the original software company. To my mind in some ways, Steam is a bit like the bargain basement areas of department stores of old.

Its not dissimilar to Microsoft programs like Flight Simulator 2020 and later that require the user to be logged in to MS in order to use that program. In fact, with the latest version of Flight Simulator as already publicly previewed is that most of the program will exist in the cloud rather than on one's computer.