Hi
VP is not really designed to do this sort of thing but provided the area to be obscured is moving in a relatively straight line in the clip concerned it is quite possible but needs patience to set it up.(and provided VP doesn't crash!)
To start with.
Place the clip with the part to be obscured onto the main sequence track.
Create a blank frame (black or white depending on your fancy) and drag and drop this onto the overlay track.
Expand the track.
Select the overlay frame and drag it left so it is coincident with the start of the main video. Now drag the right hand end out so it has the same duration as the main track. (Or you can go into storyboad mode and set the duration there.)
Place the sequence cursor at the start.
Now.......
Right click the overlay image "Add or Edit Video Effects" star followed by "Add Effects"
Move the "Appplied Effects" window to the left so you can still see the sequence preview screen. It should display your overlay frame at this point as your main video is covered by this.
Select the "Motion Effect"
The next step is to reduce the size of the blank frame so it is the same size as the part to be obscured. So move the "Scale%" slider to the left until the blank frame is the size required. At this point the aspect ratio of the rectangle will remain unchanged. Assuming you have selected black for your blank frame you should end up by seeing in the sequence preview window on the right (behind the effects window) the first frame of your video with a black rectangle in the centre.
The next step is to move this black rectangle so it covers the part of the video frame required.
Do this by altering the "Position X (%)" and the "Position Y(%)" values. (Be patient and do it a bit at a time!)
Once you have done this click each of the Green Crosses to fix the position of the overlay for the first keyframe.
Click "Close" to go back to the main VP window and slide the red cursor along to the end of the clip. (You will see your black rectangle remain staionary at this point.
Open the overlay "Motion" effects window again (click the star etc.) and slide the effects red preview effects cursor along to the end of the overlay clip. (The overlay preview will not change as you do this and will appear as a black rectangle surrounded by a chequered area.).
The first keyframe reference points will turn green and remain in place when you do this.
Now repeat the positioning procedure once again with the Position X/Y% values using the last video frame visible in the sequence preview window on the right.
Fix the new values for keyframe 2 with the Green Crosses and press "Close"
When you now play the sequence you should see a black rectangle move with the video and covering your "unwanted" area.
(This is provided VP doesn't crash......which it did on several occasions during the positioning routine.)
It did reload the auto-saved project each time however...
Anyway that's the procedure. It works best if the object you wish to cover moves at a constant speed in a straight line.
If you find the the black rectangle a bit too "abrupt" you could add a "Smooth" effect to it which would blur the edges.
Nat