Does anyone know what the thickness is of the standard VPI felt mat?

Everyone should have one of these alignment/blank side discs, 2mm thick. Alignment grids one side for overhang, 2 null points. Blank side to set anti-skate visually.

They can be used to raise a thin mat if tonearm height is not easily adjustable.

Huh...mat on a VPI?

And I always believed LP on bare platter was the rule for great sonics.

The VPI police will be  notified.

The newer VPI turntables like the original Prime come with a felt mat with the VPI logo on it and strobe markings.  No need to bother the VPI POLICE. 

no mat for me

After an A/B test, I have decided to go without any mat.  Much less of a hassle and to me it sounds better!