Indeed, I wonder why ATC didn't suggest it.
I read through the script again and it looks like the controller should have advised a northern heading with a climb in order to get both vertical and lateral separation. The controller said to turn left heading 180 immediately, and the Viper began a turn to the south but that only seems to have put it right in front of the Cessna, whereas a right turn to 360 would have put it further away. One could say that when ATC told the viper that the traffic was passing below at 1400 feet, the viper should have climbed immediately since it was only at 1500 feet, but the collision occurred 2 seconds later so who knows, might have been too late. Either way the altitude readings were off, maybe a fault on the Cessna's mode C or an altimeter setting issue. Frankly, the vector to the south with no altitude increase is very strange. I guess maybe if the viper banked 90 degrees and did a 9g turn to 180 it would have avoided the Cessna, but a northern turn with a climb would have made a lot more sense, especially since the Cessna's track was 110 (slightly south).
Edit: To clarify what I meant about the 9g turn, ATC said turn 180, and the viper made a total of 45* of heading change (from a 260 to a 215) over the course of 18 seconds, which is just 3 seconds slower than a standard rate turn of 3* per second (probably 3 seconds of looking for the Cessna) which is normal for instrument flight. 18 seconds might seem like forever for a 9g capable aircraft to turn just 45 degrees, but he was flying instruments and being vectored by ATC, not performing an airshow.