I was having strange anomolies with my Logitech cordless optical mouse in Vegas 4, and tried all the work-arounds posted here. My cursor would speed up dramatically, forcing me to use my Wacom tablet to use Vegas.

There was one work-around which I did not see posted which worked for me, and allowed me to keep the latest version of the mouseware drivers installed.

Here it is:
Go to Mouse Properties via control panel.
On the Motion tab, under acceleration, uncheck the "disable acceleration in games" option.

My problem went away after this change.
Good luck
warren