The best time for the proposal is two hours before sunset. You would have time for taking pictures around the city in the beautifiul and soft sunset-light and you can go for dinner straight after in a typical restaurant.
Nice locations for propose in Venice are: a gondola-tour close to Rialto Bridge or Piazza San Marco, San Giorgio Island (less people / having Piazza San Marco in the background of your proposal-photos).
Nice locations for propose in Vienna are:
Riesenrad (big ferris wheel), Burggarten, Türkenschanzpark, Schloss Belvedere.
Available in Venice, Vienna, Europe. © Michele Agostinis