My laptop ran out of juice last night. I plugged it in and Vegas pops up, already open. Solid state drives are able to pick up where they left off, and don't need to reboot the whole thing. I've restarted it like this a million times before and it's never had any problems. Even with vegas in the middle of rendering (which it wasn't, this time), it picks up right where it left off when you plug it in. Anyways, I go to edit a film grain effect, and it only lets me type 1 or 2 numbers for the slider value before the numbers field just flashes and the cursor disappears. I can't put the value as 0.75, and I can't even copy and paste. I tried restarting the computer, restarting the Vegas, and re-installing Vegas. It is the only thing that has any problem what so ever. I made the account just to ask. I hate it when I have some little problem that seems like it should be an easy fix like this, but it's like no one else in the universe has ever had this problem before and no one answers. It makes me feel stupid and alone. On top of that, functionally, it's very frustrating.
I had something similar happen in another application a long time ago. The problem was a corruption in the user profile in Windows. Try creating another login account in Windows to force a new profile and run Sony Vegas when logged in as that new user account. If the problem still exists, then at least you know it is not related to the user profile.