I took this itinerary a few years ago with Princess. It was much cheaper than going to the same places by land. We had a balcony which I highly recommend because you are "at sea" a lot.
The amount of time in each port is very limited, so be sure to plan ahead. You can't be impromptu on this trip and feel like you saw anything. The bike tour in Tallin was one of trip's highlights, as was the trip to the church organ recital in Poland.
I am also vegan. It was HARD! I received a menu the day before, but hardly anything was truly vegan. Even the vegetarian food was awkward, and once it was actually inedible. How anyone could do what they did to perfectly innocent eggplant is beyond me. Since I am not vegan due to moral issues, but health ones, I ate the grilled salmon a lot. It was the only way I could get protein on the ship. No beans! ;)
I also spent an inordinate amount of time on shore eating. I was hungry most of the trip. There is a fantastic veg restaurant in Gamla Stan (Stockholm) that is in Rick's book. All of their dishes have a vegan and a vegetarian form. They serve the food on platters piled several inches high. I did everything but lick the platter at the end. LOL
But if you are vegan and travel a lot, you are used to being hungry some days. I kept reminding myself that a couple of weeks is not long enough to starve to death or become malnourished. I needed to lose a couple of pounds anyway, so it worked for me. ;)