Just food for thought, look at the following alternatives to a Nikon branded flash (although, I don't think you can beat the performance of the Nikon Speedlights for less money...you can acquire good flashes for much less monkey, but don't kid yourself into thinking they are as powerful or feature-laden)...
1) Nissin i622
2) Sigma 530 or 610 ST
3) Sunpack PZ42X
All of these are SUPPOSED to be Nikon iTTL compatible, which you can't say about the Vivitar, although Vivitar makes some solid flashes for the money. I left out Metz, even though their products are awesome, simply because you can buy the Nikon Speedlights for the same or less money.
I haven't used the Nissin yet, but am hearing REALLY good things about it for the price...