redtippman and Dao covered the most significant difference. One feature NOT mentioned is that the Vivitar 285HV has a built-in PC connection. The Vivitar also has a mechanical, analog-style circular calculator dial that shows multiple info points at the SAME time, which is something an LCD system simply cannot do.
The Vivitar is, in some ways, a superior tool for the experienced worker who understands the value of a flash that is NOT directly connected in lock-step to a camera's light metering system.
The biggest PITA with the Vivitar though is its inability to swivel...all it does is TILT, so it needs to be connected with a TTL remote cord or put on a bracket if one wishes to shoot talls with bounce flash, or to get the flash bounced off of say, the wall to the left or behind and above the camera position. For off-camera use, I prefer the Vivitar 285HV to many other flashes. I have three of them, and really have used them a lot over the decades. I bought the SB 600 for my wife with the D40 back when that was new...the 600 is a nice, sleek, small flash, less-bulky and clunky-styled than the Vivitar. It works pretty well as a TTL flash.