Based on tests, it works fine to encoding realtime on my system with x264 vfw methods on medium settings at 640x480.
Instead of 640x480, you should be using 720x480 non-square encodings. You lose horizontal resolution bt doing 640x40 square pixels.
When I got D-VHS decks in order to originally capture OTA broadcasts via firewire from the STB, I ended up with a bonus feature... the ability to feed the D-VHS deck analog and have it convert it to 15 Mbps 720x480 MPEG-2 that I feed in realtime to the computer via firewire. The only processing I do beforehand is to send it through my professional TBC in order to remove NTSC setup, as MPEG-2 video does not have setup.