Is the Vivitar VIV-SF-4000 flash compatible with a Canon Rebel XT? It will flash but doesn't seem to sync with all the settings.
The SF-4000 is a manual-only flash. If you look at the foot, it only has a single pin. This isn't unreasonable for a $20 flash unit. It will fire in sync so that the light will register in the image when you take the shot. But it won't understand 2nd curtain sync, HSS, or eTTL signaling, and it cannot be controlled from the camera menu--that type of communication requires the other four pins of the Canon hotshoe (look at the camera hotshoe contacts).
The SF-4000 does also have "dumb" optical slave modes. It does not "speak" Canon's wireless eTTL optical slave protocol, but does have a sensor that will fire the flash when it senses another flash burst, so you can use it off-camera by tripping it with your XT's pop-up flash. However, the XT's flash cannot be put into M mode (i.e., not fire an eTTL pre-flash), so you'll have to use a slave mode on the SF-4000 that can ignore the preflash for the proper timing.