I use Virtual Dub very extensively and is my primary and sole video editor and I know its ins and outs very well.
There is an option in the compression dialog that, upon choosing the codec, you can configure it. Some codecs have the "force key frames every x frames" field being accessible. If you want to snap to the second, you'll need to know the frame rate of your video ("video - frame rate" will show this). Round the frame rate to the nearest integer and put this value in the "force key frames every x frames" field. Unless it's a customized video, you'll usually use 30 or 25 (NTSC or PAL respectively), 24 if it's a film.
If you want to move between single frames, use the arrow keys. Press right to go forward a frame, left to go back (going back is often slow if the last key frame was several seconds ago). Hold the shift key to snap to key frames (known as I frames in MPEG and H.264 videos). This keyboard on sliders trick works in pretty much any program to make very fine adjustments. When the starting point is not a key frame and the video is in "direct stream copy" mode, the previous key frame is used.
I utilize this mechanic to edit any of my videos (where key frames can even be 10 seconds apart) accurate to the frame.