Which methods have you tried?
Seems like I can do it without a red bar this way:
- New title (Name it something like "vignette")
- Draw a rectangle using the rectangle tool
- Change the fill type to radial gradient
- Make the left colour white, and the right colour black or a shade of grey
- Adjust these to get the size that you desire
- Close the titler window and drag the title to the timeline above your video
- Click the title
- Click the effect controls tab
- Click opacity
- Set the blend mode to "multiply" (Or one that looks how you want it; "colour burn" looks different)
- Adjust the opacity to get the darkness of vignette you desire
If you want to adjust the size, double click the title and adjust the colour sliders.
If you still get a red bar, maybe CS6 or above has changed (I tried this using CS5.5) or you don't have CUDA turned on.