Here are some of them:
1. When trying to write a video onto a file that's in read-only, there's no "Try again" option; it just fails and the user has to start all over again.

User error, don't try to render to a file that's read only.

2. After a failed render or after another edit, the name of the file previously entered is gone and the user has to enter it again.

Oh dear, is it that hard to copy paste.... Why the failed render..? User error, poor hardware ?

3. Clicking "Close program" by mistake and then clicking the window’s X (expecting to cancel) closes the application anyway. This is diabolical.

User error, don't click "Close program" by mistake.

4. Exporting with custom frame dimensions requires far too many clicks. Simply enable the Width and Height fields by default and, when you enter a number, it automatically jumps the "Frame size:" field to (Custom frame size).

I guess you want it to read your mind.

5. What in god's name is going on when you're using the mouse-wheel to zoom out of the video preview? The camera position jumps all over the place.

No problem here, (9:16 is awkward tho)

6. Handling video clips that are on top of each other is quite frustrating. If two clips are on top of each other the "fx" and "..." menus for one of the tracks often just disappear and it's impossible to select the clip underneath.

Why do you put videos on top of each other? There's an option called Takes.

7. Many UI elements that adjust rectangles (masking, pan/crop, event position) resize symmetrically around a center point instead of just moving the edge selected. This makes applying a mask (or any other action involving this UI) very cumbersome. The UI to adjust the clip position in the Video Event FX UI... I don't even know.

Picture in Picture, masking & Pan/Crop will resize around the centre, grab the corner points or highlight the 'Size About Centre' in Pan/Crop.

8. This is a big one: The mouse-wheel zooms in on the timeline at the playback position instead of the mouse-cursor. This is one of the most infuriating things about navigating Vegas Pro.

That's a positive to me not a negative, I don't want to have the timeline zoom relative to my mouse cursor, that'd be a pain having to position the mouse before zooming.

Again, this is about removing entirely unnecessary friction from the user experience without sacrificing any of Vegas Pro's power. I don't believe that these are intentional design choices, so I am wondering how some of these haven't had the necessary priority to be addressed in all the time Vegas Pro has been on the market and why its users are ok with them not being fixed.

Learn to use the software.

I apologize if this seems harsh but this is prob 'why its users are ok with them not being fixed'

Well explained @Gid 👍

User error, don't try to render to a file that's read only.

That's the most moronic take I've read in a long while.

I guess you want it to read your mind

No, I want it to not waste my time.

I guess I see the problem now and why these issues are not being fixed. Its user base is made up of elitists who take pride in being able to cope with terrible UI. Thank you for the clarification!