I just updated the post again with a 1270 track AU3 template that I recommend everyone start out with. Read new thread as of 1 minute ago to understand the issues.
New thread here: viewtopic.php?f=9&t=143416
To your question....
If you are content to never use more than midi port 1, then I see no reason AU2 wouldn't be fine. AU2 also supports the undocumented aux track trick, which AU3 does not in Logic Pro as of this writing.
With AU2, as you know, you can have up to 16 channels per VEP instance, so that could be 16 instruments or one instrument with up to 16 articulations, which is probably how you tend to use it in your one-track-per-vep-instance mode of operation.
On the other hand, aside from the missing aux track trick, I have had no problems with the AU3 plugin and it does support up to 127 midi channels per VEP instance through 8 ports, so that gives you options for more articulations, or more instruments, etc.. It is for now slightly laborious to setup compared to 16 part multi-timbral, until Apple will hopefully figure out they need to give us the option to create up to 127 multi-timbral parts from the new tracks wizard. But that should not effect you at all if you are constrained to 16 parts anyway.
I do think AU3 will be the future for VEP and I see no problem in using that version as long as you don't need the aux track trick.
VST is not applicable to LPX. In Cubase S1 and other hosts, you can use VST3 to get 768 midi channels per VEP instance.
5,1 MacPro 3.46ghz x 12 128gb ram, OSX 10.14, Logic Pro 10.4.8, Mainstage3, Cubase10, StudioOne, Reaper, DP9, VEP, VSL, too many plugins to list