I've been attempting to run two video tracks side by side as a split screen. (Its actually two motorbikes on the same racetrack to compare lines/styles etc.)
Although both of the individual clips play quite happily in Videolan and import into Vegas OK when I run the split screen it becomes incredibly jerky.
The split screen effect was achieved by using two tracks and the track motion feature of each track to just use half the screen, one right, one left.
This is on a brand new Sony Vaio VPCCW1S1E and I've tried it with the clips stored on an external hard drive and on the lappys hard drive.
Could this be that Vegas just requires so much processor power to perform this that my laptop isn't up to the task? (Certainly I hope not!).