Perche already posted most of what I was going to say. Take it easy and enjoy a vacation, not a dash across Italy. If it's your first time and you have 8-10 days, do you have the option to fly home from the last city you visit, or do you need to backtrack to Bonn first? If not, I'd suggest just moving up or down the rail corridor in Italy as your schedule permits. Fly into VCE, spend 3 days in Venice, then get on the Freccia to Rome and spend the remainder there, fly home from FCO. Or the reverse.

I'm a big fan of Florence, such that I just spent 8 days there. It is in between Venice and Rome on the rails. If you have 10 days, doing 2 in Florence might be nice, but think about whether you really want to do the whole luggage/cab/unpack/repack for 1 night at a hotel. I'd skip it unless you could make it 3 days.

For Rome, I'd say 4 days minimum if you really want to see and experience it. Otherwise, it's just a dash from place to place. Rome is more spread out than Venice or Florence, so covering it all in a day or two is tough unless you're really just running around with a checklist.

Ultimately, just don't expect to do it all in any one city. My first time to Italy worked out that way. Our return visits and plans have been 1 city for 7-10 days. But still, don't stretch yourself too thin or you won't have any idea what you even want to go back and revisit.