How do you install a 12" VPI armboard for 12" Unipivot on a VPI Classic 3 - Thanks :-)

The most important point is that you are very happy with your revitalized TT.
And it sounds like VPI stepped up as far as customer service is concerned which is great stuff.

As your name here says.... Have vinyl fun!

I am fortunately definitely having a lot of clearly. very improved vinyl fun!! 

I've also made two other VPI turntable improvements you may wish to consider for yourself :

1) Replaced the four VPI Classic feet with Isoacoustic GAIA 2's.
  GAIA"s come with three thread options, including the 1/4" 20 you need.
  In brief, their added vibration damping/control clearly gives my music      lots more clarity, detail and space...it was obvious...wow!

2) The original 10" tonearm lift could not reach up and lift the 12" 3D tonearm, even after using an Allen/hex key to make the max adjustment.  Adding a pad or similar on to the lift arm was needed.  The solution?  To be found at that well known, high-end audiophile store...Home Depot...I kid you not...cut to size some double sided tape, place on the lift, then cut to size and trim the height of a 2" circular black rubber washer to go on top and you're done.  More precisely "Scotch" brand "Mount" 15lbs strong tape and an "Everbilt" brand 2" flush valve shank washer...yes, the washer is originally meant for a toilet but it works great :-)

I have just now added a $150 VPI Dual Pivot Tonearm Adapter to the base of the 12" 3D arm.   The net impact of this "upgrade" in my VPI vinyl system - with the Ortofon Quintet Black S - is it clearly sounds better.  Yeah!   All together, I'd say the net impact is around a +20/% or so improvement, with all the impacts/changes assessed together. 
There is:

1) More clarity and crispness between instruments AND more clarity and crispness between all the instruments and vocals,
2) Deeper, clearer bass end AND more detail in the bass sounds.  For example, I can now hear the strum "journey" of a bass guitar string and not just one broad or slightly smeared bass note.
3) A slightly less airy, floating top end.  The range of the top end is shorter.  To my ears the degree of this change is so much, much less than the improvements in 1) and 2) above.

Very happy I did this.  :-)