I'm thinking of porting my vis to foobar so I had a quick look at the vis_test example.

In the vis_test the 'rendering' is performed in the on_data function,
but I'm wondering if that would be the right way to get a 'stable' framerate.
In general, how often is the on_data function called (per second) ?

The number of calls per second is specified by the user in the, "Visualizations," panel in the foobar2000 preferences.

I thought that was for the spectrum/waveform display in the userinterface;
If i set it to 5 the G-Force-plugin still runs at full framerate...
Or was the GForce-plugin not made as a vis-plugin but as a custom-plugin..?

G-Force for foobar2000 definitely is implemented as a normal visualization. However, it seems to copy the wave data it receives and perform its own FFT (for the spectrum). Setting 5 FPS in the Visualizations section only means that it will get new wave data 5 times a second (and larger chunks than it would get at 30 FPS). I don't know for sure, but that would make the most sense for me.