The motion sensors send an "off" command after a certain time period. You can turn the camera on manually, but if the sensor trips, it's going to send that "off" after a while. If you get Active Home Pro, you can get around this using macros to monitor the sensors, having the cameras on a different address than the sensors (say A3 and A4), then letting the macros control the cameras. I assume the "PC Transceiver" you have is a CM19A. If that's correct, all you need is AHP V3.271. There might be a way to do this with Vanguard, but I know nothing about Vanguard.
There is currently a "deal" on AHP (as of 7/10/2010) for $10.00 at
http://www.x10.com/promotions/sw31a_ed_ir_0628_v2.html and you get another CM19A and an IR Commander with it. Don't let them tell you that you need another transceiver - your TM751 will work nicely.
** BEWARE ** - you need the Smart Macros plugin with AHP to do what you want to do. Even though the deal mentions
Add IR control to ActiveHome Macros for the FIRST TIME EVER!
it doesn't show the plugin in the "what's included" section! You might have to pay extra to get the plugin! If that's the case, you might want to search X10.com for a deal that includes ALL the AHP plugins.
Hope that helps!
ITguy