Search the forum for 'venice', that has been discussed before.
The trains are modern, comfortable, free WiFi, power sockets, and a bistro-restaurant car.
Trains leave every couple of hours through the day.
2. Now either:
a) Stay overnight and take the 06:25 EuroCity to Venice arriving 14:05. Fares for this start at just €29 booked at www.oebb.at . The train takes the wonderfully scenic UNESCO-listed Semmering route, it's a gem, a fab journey, see https:/…Semmering_railway
From 10 December 2017 this once a day EuroCity train will become two daily Railjets with bistro-restaurant, one leaving Vienna at 06:25 and another at 12:25, a significant improvement!
b) Catch the sleeper! A Nightjet sleeper train leaves Vienna at 21:27 arriving Venice Santa Lucia on the banks of the Grand Canal itself at 08:24 next morning. Saves a hotel bill and a sleeper ride is a unique European experience.
Book at www.oebb.at from €59 with couchette or €99 with a bed in a cosy 2-bed sleeper, breakfast included. You print your own ticket.
You could leave central Budapest at 15:40, have dinner in Vienna and be on the Grand Canal in Venice at breakfasttime next morning. Who needs planes?