Here's a theory: when your Vista PC goes to sleep (as opposed to hibernate or hybrid sleep), everything is still active in RAM but put into a low power standby mode. When the PC wakes up, everything comes back, and the graphics display needs to be turned back on. I believe going to sleep and coming back is the responsibility of the BIOS, and where the display is involved too, the graphics card driver.
When your graphics card goes to sleep, your BIOS and your graphics card driver are supposed to be smart about it. When waking up, the least clean way to restart the graphics card would be to reset the card and then load up every saved setting one by one. This is the least clean because every setting change on the card while it shuts down, starts up, changes resolutions, etc. is going to make a little disturbance on the video output. It's possible this is what your graphics card is doing when you see all that flickering. The way the system is supposed to work is that your graphics card supports some kind of low power mode with rapid restore, and your BIOS and graphics card driver is supposed to know how to use it to wake up from the sleep state with a minimal delay and flickering.
The bottom line is that the excessive flickering may be the fault of your BIOS or the card's graphics driver, and you might want to see if there are any updates for those.