it has to do with the components used within the PSU
the VS is a basic model and uses cheaper parts (most likely Chinese, maybe a few Taiwanese caps vs higher quality Japaneses in the TX) that are rated for lower temps then the TX (i assume you mean the TX v2). The VS also offers less efficiency (the ratio between the useful output of an energy conversion device and the input), offers less connecting cables and smaller warranty.
basically, the TX 650 v2 is designed to deliver 650w (or more) consistently at high temp, efficiently, to a gaming PC with dedicated video card(s) that draw a lot of power
the VS is designed to replace a PSU in a mass produced PC like Dell, Acer, HP, etc., While it may peak at 650w it will have a difficult time delivering that power consistently, efficiently, and at the same temp (your PC internal temps can exceed 50c while pushed) as the TX.
this article may help you out, while the VS is a step up in quality from the techsolo unit, it would be ranked below the OCZ unit in terms of quality platform. The TX v2 would rank equal in terms of quality with the Antec unit (in fact both are made by the same OEM)
http://www.anandtech.com/show/3985/three-550w-psus-for-different-prices