Hi Stacey, welcome to the forums.
I used an old flash with my K10D, a Focal M500CS-ZOOM, which I guestimate to be around 20 yrs old, and it worked fine. I did grab my voltage meter and ran multiple tests to verify the firing voltage was acceptable. Some flashes were known to vary from the normal voltages for the model/line so I didn't want to take any chances.
If you're going with non-pttl flashes, to get the best results you will want to switch your camera to Manual mode to use these types of flashes, old or new ones. That Sunpack should be fine power-wise but unless I'm mistaken that flash will not talk to the camera so you will have to go manual for best results.
FYI - Cameramanuals.org and butkus.org have tons of old manuals at hand.
No luck on the manual for the Vivitar 1900 but here's a link to the Vivitar 283 flash manual
http://www.cameramanuals.org/flashes_meters/vivitar_283.pdf