I've done what you want by accident. Try putting a copy of your PIVlab session .mat file somewhere else, then opening it. When PIVlab says it can't find the images, just click cancel. You'll get the vectors but no background image.
There's probably a better way but this will get you what you want in the mean time!
Hm, I don't know if there is a better way... You could also try to display a derived parameter (e.g. vorticity), then set the min and max values of the parameter somewhere outside of the data range. Then change the color map to grayscale. That should give a uniform background color.
Or just export the image as PDF, open it in e.g. Adobe Illustrator and delete the image map.