A few months ago I started having problems viewing the preview window in a split screen with one half bypassing plugins and effects because it would flicker and rapidly cut to other frames and clips.
Then I had a problem doing it on a video I had to do for work where I had put text on a video track above a shot. It never would play right until I disabled GPU acceleration. My graphics card is an Nvidia GeForce GTX 450, which is old, underpowered, and was having some conflict with Vegas.
For the first time in months I can view my preview monitor with plugins bypassed properly.