With Analog, you have to manually start & stop. When capturing from DV tape, you can use batch capture to set start and stop times. The reason analog is manual is because it does not have a timecode.
I too use a ADVC-100 to capture analog. I created a Visual Basic Macro in Microsoft Excel to start and stop Vegas Video Capture. If you have Excel loaded on your PC, let me know if you would like a copy. It's not pretty, but it does work. It emulates starting and stopping as if you were using your keyboard ('SendKeys' command in Visual Basic).