[Electronic M]

From how I understand it most tubes needed more than 6V (or 12 in newer cars) for the B+ supply. A Vibrator form how I understand it is like a relay wired so that every time it's coil de-energises the contacts close and re-energise the coil. Thus causing it to oscilate. Coils detest sudden changes in current and create a large magnetic field to fight/counteract the change, so if you put a suitable secondary coil in the field generated by the current pulses you can harvest non-sinusoidal AC power from the field to feed the B+ circuits, but it must first be rectified and filtered just like any transformer type home set.

And that is why rectifier tubes were used (though i've heard of self-rectifying vibrators which may or may not have an internal rectifier of some sort).