This works in Final Cut Pro, not sure about imovie:
Find a friend that has a DV camera with composite inputs on it (people with kids almost always have one), connect the VHS deck to that and then connect the DV camera via firewire and capture in free run mode (no tape in the DV camera). As long as you see the VHS in the viewfinder of the DV camera, you should see it in imovie.
imovie will think that it's recording from the camera, but will actually be displaying the VHS.