Why the ion cannon and not the tractor beam? WIth that many shots hitting you'll get a LOT more mileage out of the tractor beam. Also, Crack Shot on Vess; getting him up to PS8 is a lot less important than forcing that critically important hit through.
Ion for damage and control (Vessery has always liked being able to ionise, K-Turn, and then walk someone off the board; TIE/D makes that even better). Plus the other three ships can take advantage of an ionised ship as well, potentially moreso than TBeam (although not necessarily).
Also VI is there because when he lands the Tracers, the TAPs get TLs which triggers their titles so they also get Evades. The sooner they get Evades the more likely they are to survive that first round and get off the missiles. Any unfired ordnance is a blow against a list, so that's a priority. It also gives Vess an edge against non-VI Brobots by default, which will go a long way in helping with that matchup.
I'd question ions in a list without a way to get two on a big ship in a single turn. And I think you're underestimating the value of -1 agility.
Getting VI just for a single evade on each TAP slightly earlier (but not early enough to make the difference against PS9+) seems like a waste to me compared to forcing through a hit on the likes of Soontir or Poe. Vessery's arc with the Tractor Beam (from experience trialling it since the card was spoiled) is a death zone for small ships when the rest of the list has a lot of shots to spend, and not a pleasant place even for large ones.
The predictability of ions is great, but I'm not sure it's worth spending two points on.