Hi Frank,
I am extremely envious of your travel plans. I hope you have a wonderful trip!
This past summer I chose Verona as my home base for northern Italy and was very happy with my choice. It’s a beautiful town and allows for quick trips to Mantua, Vicenza, Padova, Venice, Lake Garda and Milan.
Most importantly, in my opinion, it’s very quick to get from downtown Verona to the train station for day trips. Sleeping in Venice is lovely, of course, but it takes longer to get to the train station (and Venice is generally more expensive). Padova would be another good home base, as there is plenty to do and the train station is well-situated (and is closer to Venice, if you plan on visiting there a couple times).
Milan is a great city, although less central if you are interested in any of the above towns (and will take longer to get to Vienna). I guess it depends on what experience you want. After Rome you might be in the mood for a smaller town.
No matter what you choose, I’m sure you will have a great time!
Rebekah