In general, none of the Vult modules are tested for sample rates less than 44.1 kHz. I do not have plans to support less than 44.1 kHz.
If you lower the sample rate expecting to consume less CPU, in most cases that will not be the case for Vult modules. Many filters become unstable at low sample rates, that requires me to implement internal over-sampling in order to keep the filters from failing. So, even when you set 24 kHz, the module will be running at 44.1 kHz and doing resampling in order to get a 24 kHz output. At the end, you will consume more CPU than simply running at 44.1 kHz.
The work of adding support for lower than 44.1 kHz in all modules is just too much for me and there’s no real benefit for users (since there will not be reduction of CPU consumption).