There are ways to do what you want to do, but it will be a bit of a hassle and you will lose picture quality. You're better off re-capturing your VHS tapes.
If your camcorder does analog-to-digital conversion, then it will be easy. Connect your VCR to your camera (probably using a component to mini-plug cable, but check to see what your specific camera uses) and then your camera to your computer via firewire. Set your camera to playback mode and launch iMovie. You should be able to capture what's playing on the VCR straight to your computer.
If your camera does not support analog to digital conversion, then you can just break the process into two steps: first, capture from VCR to miniDV tape in your camera (use the record function instead of playback; think of old school cassette tape copying), and then, when done, import into iMovie as usual. The downside to this method is that it takes twice as long and you can only record an hour at a time.
