Welcome to OG Greg. I know you'll enjoy it here. There are a great variety of shooters and hunters who frequent this forum, who are willing to share their knowledge and experience.
I have the 3" Vinci and shot it pretty extensively with a variety of chokes and shells. I've been a Benelli shooter since 1991 when I purchased my first SBE. The inside diameter of the Benelli barrels is, as has already been noted, is generally quite tight. I have one barrel for my SBE that has a .719 i.d.
Again, as has been noted, chokes with exit diameters of .640 - .655 have always worked pretty well but e.d. is just one part of the overall geometry of the choke tube. It's really all about what's inside that choke that makes the difference.
The SSX (.643), the Wright's #3 (.650) and the new Hevi-13 turkey choke (.667) all work very well for me in my Vinci. (That H-13 choke is a real sleeper and seems to the exception to the .640-.655 rule.)
With your 3.5" Vinci, I'd still recommend that you shoot 3" shells. They do the job, you don't gain any range with the 3.5" shells and it's just much easier to get back on target with the 3" shell. (The Vinci has less muzzle jump than any gun I shoot.)
I'd recommend the 3" Hevi-13 "Bronze" shells with 2 oz. of #6 or #7 shot. They are great shells and I've found them to be very consistent performers.
Last August, I wrote an article about my first 30 days with the Vinci. While it is the 3" model, you may find some of it to be interesting to you.
http://allaboutshooting.com/article_info.php?articles_id=418You have a great gun and whatever choke or shell you choose, I know you'll enjoy using it in the turkey woods as much as the duck blind.
Thanks,
Clark
