Some older 285, 285HV and 383 Vivitar flashes have very high sync voltages, some higher than the Rebel's electronic circuitry can handle.
Photo Strobe Trigger Voltages
A Wein Safe Sync will protect your camera circuitry and also provide a PC sync point.
Safe Syncs
Also be aware that the Vivitar units used their own proprietary sync cords, not just a male-female PC cord like many flash units use.
The 283-285 series flashes from Vivitar are good general purpose flashes that can be had for peanuts used on eBay and other places. There is also a wide variety of accessories available for these units that also can be had very-very inexpensively.
You can get good exposure using a Thyristor based auto flash as long as you remember to use the computer that you have between your ears.
My only gripe about the 283-285 (series) units is that they do not swivel, so bouncing using the flash on a hot shoe is not always easy. However, since you are going to be using a sync cord anyway, splurge on a flip-camera flash backet which will colve the lack of swivel problem because the flash will always be pointed in the same direction for bouncing whether you are in vertical or horizontal position.