I believe you’re seeing overscan lines which in my digitized VHS appeared as lines on the bottom and narrow bands of distortion on both sides. I eliminated the distractions by using the crop tool in iMovie.
Thanks both. I think I'll do something like what you recommended. I definitely have the top/bottom distortion but also some side-to-side and color bleeding too. They're just overall bad quality, unfortunately. Probably the original medium is in bad shape. I can try to get an actual converter too (I had a VHS player hooked to my camcorder hooked to composite input on a capture card).