I agree that "stupid" pre-flash is annoying for us cavers, but it does serve a useful purpose setting the white balance to theoretically help the average Joe user take better snapshots. Anyway, it would seem that most of the newer Canon digital cameras have a hidden method for turning off the pre-flash.
When you select the manual mode setting on both my Canons (the A540 and my new S3 IS) the camera automatically goes to single flash, enabling the use of regular slave units for cave photography etc. Plus you have the option to set that single flash to 1/3, 2/3, or full power.
Wendy, I think you have a Canon too - right? It should be the same for you.
Sorry
I know this is a little off-topic for the Vivitar 285. I prefer trusty Sunpak 383's myself.