Want to travel to eastern europe in july2016

You really need to limit yourself to 2 or 3 cities.

Too many cities...too little time.
You should do no more than 3...Vienna - Budapest - Prague - Vienna...cut out the other two for this trip.
Assuming 8 nights...7 days.
Day 1 - Arrive Vienna
Day 2 - Vienna
Day 3 - Vienna
Day 4 - AM Vienna...train to Budapest
Day 5 - Budapest
Day 6 - AM Budapest...PM train to Prague
Day 7 - Day in Prague; PM train back to Vienna
This itinerary gives you 1.5 + days in each city...enough to get a feel for each but you'll want more! If you want to cut it down to just 2, I would eliminate Budapest and add a day each to Vienna and Prague.
Trains will be easiest to go between these cities. The earlier you purchase your tickets, the cheaper it will be. You are already quite late to be purchasing tickets for the summer season but it will be even more expensive a month from now as the cheaper tickets are bought by others.