I emailed Pentax and got this (very fast) reply-
Dear Sir/Madam,
Thank you for your enquiry.
The only fully compatible flashguns are the AF360FGZ and AF540FGZ, all other older flashguns do not support P-TTL.
Unfortunately we do not know the specifications of your Vivitar flashgun.
Please make sure the trigger voltage does not exceed the 30 Volts else you might damage the camera.
If the flashgun meets this requirement the flashgun might work. Please keep the following in mind:
(P-TTL stands for Pentax TTL and not Preflash TTL as many people think. This is similar to Nikon's i-TTL and Canon's E-TTL)
When the flashgun meets the requirements of having a trigger voltage below 30 Volts, this flashunit can be used but it's not fully compatible because this is an A-TTL flashunit instead of a P-TTL flashunit.
This means that the camera and the flashunit cannot synchronise properly with eachother.
TTL dedicated flashes for Pentax use a separate metering sensor in the light box to get a reading from the reflection of light from the film (or sensor) plane during the exposure. The problem with this type of system is that the CCD sensor in digital bodies is more reflective than film, so calibration is different. The major advantage is that with only one flash, there is less chance of "droopy" or closed eyes from fast reacting people or animals, and standard slave flashes will work without any problems.
P-TTL dedicated flashes for Pentax use the regular exposure meter to get a reading before the actual exposure using a pre-flash just before the shutter opens. You can actually see the preflash in the viewfinder just before it blacks out.
Because of the pre-flash, only "digital" slaves that fire on the second flash will work correctly with a P-TTL flash unit.
TTL Auto Flash will not work. You have to use manual flash settings and use the flash aperture control.
You also have to set the ISO on both camera and flash to the same value. This way using this flash will still be possible.
I hope that your question has been answered to your satisfaction.
Should you have further questions regarding this matter, please do not hesitate to contact us again.
Will get the voltage measured and if it's a lot less than 30V I'll use it. I also found the
Wein Safe Sync referenced on another site, but I think the money ($50) it costs I'd rather put towards a new flash.
Thanks again for the replies