From the code comment, I do see this
// Create our video capture device that uses DirectShow library. Note that
// the combination of resolution and frame rate that are allowed depend on
// the particular video capture device. Thus, setting incorrect resolution
// and frame rate values might cause exception or simply be ignored, depending
// on the device driver. The values set here will work for a Microsoft VX 6000,
// any many other webcams.
And from the specs page of VGA2USB here
I see that the combination of Resolution: 800x600 and Frame Rate 22.6-60 FPS. I set it to 30, Should be ok but I don't know why it is very unstable.
But in 3.5, it is extremely stable.