Hello everybody!
Does anybody know such an error in Vegas interface?
What is the reason and how to fix it?

Nope, but I'd be looking at your graphics card, and perhaps large fonts (although I don't believe that's the problem).

to see if it's windows, click START, RUN.
Type in SERVICES.MSC [enter]
Stop the THEMES service.
If it revenrts to normal, likely it's some crazy scheme in your display control panel.

Rebooting or starting THEMES again will restore you to "normal."

PS I disabled it, as I want more resources to go to video, not pretty interfaces...

"I have a need for speed" (Tom Cruise) LOL