Celine,
Are you flying into Salzburg? It might make more sense to start in either Prague or Budapest then go to Vienna then Salzburg. It's a minimum of 8 1/2 hours by train from Salzburg to Paris. There are many cool places in between, or you could check the discount airlines. Prague is north of Vienna and is about 4 1/2 hours by train. Budapest is east of Vienna and is about 3 hours by train. It's only 3 hours by train from Vienna to Salzburg.
I would suggest 2 to 3 days in Prague, 2 to 3 days in Vienna, 2 days in Salzburg and then pick a stop or 2 between Salzburg and Paris.
If you are flying into Salzburg you will be backtracking going to Vienna and Prague, but you could go that direction then take a cheap flight to Paris....just a thought.
Hope I didn't confuse you more than I helped.