I didn't stay on the main deck, but I did cruise on the Westerdam in November, and enjoyed it tremendously. (We were travelling in a Verandah Suite.) We'd previously cruised on the Rotterdam in 2002, and although the Westerdam is a larger ship - it still has the "intimate" feel of the Rotterdam due to its design.
From what I recall from my wandering around the ship at various times (I'm a ship fanatic, and tend to explore), the main deck is relatively quiet. Towards the end of the cruise, there's a bit of business at the front desk, but the people in line aren't making a lot of noise. The seating for the Vista lounge is located forward on Main deck (the main entrances are the two decks above)... so you won't see significant traffic entering the Vista lounge there, and I was actually down on that deck during a show, and you don't really hear too much noise from the show either. I'd likely avoid the front few cabins, though, just in case!!