The difference between the two is that you can tune the VTF-2 for maximum output (louder) or max extension (lower bass)by flipping a switch and plugging one of its two ports with the provided plug. The STF-2 has only one port and is not tuneable, but it sounds exactly the same as a VTF-2 in max extension mode.

Hsu discovered that most people who own the VTF-2 sub keep it in the max extension mode, so you're not really missing much by getting the STF-2 instead of the VTF-2.