There is a VP-1 Variable Power control accessory available for this flash. That's how Vivitar recommends you do it.
I have the 285HV which is similar, but I'm not familiar with the 283. Is it all auto? How do you use it normally?
Either you should be able to set the ISO on the flash for a faster or slower film speed to get it to over or under expose. Or there is an f/stop guide, in which case you would set the f/stop under or over to adjust exposure.