which Aero mount do I need for a 3-9 vx-r.Looks like maybe the standard will work.

I would suggest getting the extended at least.  It really depends on the scope.  

On a Nikon 2-7 the extended worked great but with a Primary Arms 1-4 the scope was so long I needed the spr.

Here is what a Leupold 3-9x40 VX-R Patrol looks like in a LaRue LT-104 in the front most slots on an AR15 upper.  I don't know how this cantilever offset or ring spacing compares to the Aero Precision mount you are considering.  Maybe this will help.  I did not need LaRue's extended mount, but might have were I shooting nose to charge handle.  I don't, and this offset and spacing works fine.  There is still a good bit of room to move the scope forward another inch and a half or so.

Ended up with the standard mount.  It worked great.  I don't shoot ntch, but  it could still be mounted another 1" forward if need be.  I will say that it was a pain like others have say about the scope rotating in mount as it is tightened.  I like the mount- solid and light.

Well done.  Nice looking rifle.