Vegas preview output is only possible to the DV (Firewire) output. To view that on a monitor you have to convert the DV-signal to an analog signal for your external monitor.
Using a dv camera or a dv recorder is one choice. Also there are many converters available - cards and boxes. One of these boxes is Canopus ADVC-100 which is an AD/DA-converter. Using this box you can capture from analog devices as well as using it for previewing out of Vegas.