Can the Vivitar 728AF be used safely with the Canon Rebel xti?

Safely, yes, because it does not exceed 6V.

Because the Vivitar 728AF is TTL-based and the Canon Rebel xti, being a digital body, is not TTL-compatible, but E-TTL, the Vivitar can only be used manually. It will fire, but there is no feedback from the camera to the flash unit. If you are willing to evaluate and reshoot every time, you can use this flash unit, but expect no more.