The Vis Generator is a tile entity added by Thaumcraft 6. It takes Vis from the Aura and converts it into 1000 Forge Energy (FE)/Redstone Flux (RF) per point of Vis and outputs at 20 FE/RF per tick. It can be disabled with a redstone signal.
— Thaumonomicon, Vis Powered Generator
One more impossible thing to mark off my list and it is not even lunchtime yet.
I have managed to generate mundane power using vis, though I had to cheat a bit to do it. It is a simple matter to turn kinetic energy into power and creating kinetic energy with vis is thaumaturgy 101. Using vis to turn an axle connected to a dynamo did not work as some magical energy would transmit through the axle and cause all kinds of problems.
In the end I designed a special linkage using magnets and some clever engineering. This linkage would transmit the kinetic energy while not allowing the vis and mundage [sic] energy to interfere with each other.
Unfortunately the nature of the linkage places some limits on the power that can be generated as some kinetic energy is lost. It draws vis directly from the aura to do this and a single point produces 1000 FE/RF though it can only output 20 FE/RF per tick.
A redstone signal will disable the generator.