What angle should I set the VTA on my VPI turntable?

You set VTA the same on any turntable....it’s not specific to the turntable, but to the cartridge. In general, you want the tonearm parallel to the record...from there, you can tweak it to your liking, cartridge ass down, or tipped forward...generally ass down, more bass and more full, tipped forward, more treble or high frequency detail....I belive you are looking for a stylus rake angle of about 92 degrees. You should also re-check your stylus down force upon adjusting VTA.
I set mine and mostly leave it alone...there are too many variables involved as far as record thickness etc...unless you want to drive yourself nuts and adjust for every record...

Hshifi is conflating SRA (~92 degrees) with VTA (~21-23 degrees). The two are interdependent of course.

Mijo, an analysis of data in the form of photos posted on vinyl asylum (I think) revealed that most “amateurs” draw the lines incorrectly when adjusting SRA with a USB camera. Thus they arrive at incorrect SRA. Personally I take it as the height of anal audiophilia even to try it. But that’s just me.

@noromance 

Raised at the back is lighter and brighter. Lower is heavier bass and rolled off highs. Find your preference in between. 

This is the most common mistake in setting VTA.

VTA should not be used as a tone control.
The correct VTA is when the information retrieval and soundstage presentation are maximised. That is when the stylus is aligned to the cutting angle used for the record and maximum information including natural harmonic structure is preserved on reproduction.

@audioguy

 In general, you want the tonearm parallel to the record...from there, you can tweak it to your liking, cartridge ass down, or tipped forward...generally ass down, more bass and more full, tipped forward, more treble or high frequency detail..
This is the most common mistake in setting VTA.

This is not correct.

VTA should not be used as a tone control.
The correct VTA is when the information retrieval and soundstage presentation are maximised. That is when the stylus is aligned to the cutting angle used for the record and maximum information including natural harmonic structure is preserved on reproduction.

@lewm , amazing that many people flunked out of geometry isn't it?
I already have the microscope and the set up so why not use it? It makes me feel better knowing I am right on.......even if the records are not:-) Trust me on this. I did not flunk out of geometry. That stereo cabinet I showed you was designed in my head. There were never any plans, just a list of measurements of such things as records and all the equipment.