In short.
vBIOS is a BIOS for a Graphics card, basically it holds information's about clock speed, voltages and some other stuff regarding GPU operation.
One can adjust those values, so the GPU could perform faster, similar to overclocking CPU?
Why? Because you can increase hash rates, and/or lower power consumption of the hardware.
It's not plug and play, so... google it, read about it, and eventually try it yourself if you want, but bare in mind that you can damage your hardware, brick your card or lose warranty...