The short answer, as well as the long one, is... No, Vegas is designed to preview to a 1394 device, either a camcorder or something like Canopus ADVC-100.
But, if you had a Canopus ADVC 100, you could Firewire from computer to Canopus, from Canopus to VCR, from VCR to Monitor. Accomplishing what you want, but you need the 1394 to an A/D converter (like Canopus) 1st.