Would it be possible to capture video playback from a Motorola (cable) DVR using only VMS (and HDMI cable) or would I need some other software and hardware?
You're not going to be able to "capture" the video, as you could if you were getting video from a miniDV camcorder, for instance.
It's possible you can transfer or copy the video files from your DVR to your computer and edit them. Although DVR video isn't designed to be edited, so you may find the results are less than you'd like.