Here's the deal ... brain power beats cpu power.
In video
editing in Vegas any modern CPU with good RAMs and disk I/O will work dandy. The motion graphics are integrated, and quite good, though without all the bells and whistles of Premiere/After Effects. What is also 'neat' about Vegas is that timeline media updates are automatic from outside software without rendering or refreshing. IIRC, Vegas will even 'adjust' stereoscopic 3D for correct alignment.
The key to video
conversion, or re-compression, in your work flow is to
avoid it whenever possible. If your project input file formats, setup, bit rate and resolution are consistent through output it does not have to be 'converted' and the processor only works on your titles, effects and transitions, greatly improving the speed of the 'process'.
The best thing for the OP to do is compare his proposed "3D HD video" to the options available in the Vegas 'smart render technology' ...
Smart render technology provides faster encoding for cuts-only projects or projects with minimal changes while providing no generation loss because video frames pass through the engine unaltered. When you render video to any of the following formats, unedited video frames are passed through without recompression (smart rendering):
DV AVI, DV MXF, IMX MXF (IMX 24p MXF is not supported for no-recompress rendering), HD MXF, MPEG-2 (for files such as those from HDV and DVD camcorders), and XDCAM EX
So ... you can see where this seriously reduces the need for processing power if you follow the recommended work flow. An AMD x4/x6 will be just dandy, as will an Intel product.