Vienna and Salzburg Advice needed.

The Roman Empire lasted 1,000 years. The British Empire lasted 350 years. The Austrian Empire lasted 800 years. Vienna was the capital. Salzburg was a hick town.

You can expect much warmer weather in April, could be rainy, though.

Salzburg's airport is rather small; there are no direct flight from Paris. How will your daughter travel to Salzburg?

There are several train connections per day between Paris and Salzburg. Most comfortable - I guess - is with one change (9:45 hours), fastest connection is 8:08 hours (two changes).

If she travels in February, the weather will be cold as it is winter. In April, the weather is much warmer and she will be able to do more outside. I would choose April, no question.

For the top/best sights, it is impossible to say as we have no idea about her interests. She needs to pick up some guide books and read them, or do some research online. Here's a good place to start - https://www.wien.info/en/sightseeing and https://www.salzburg.info/en

Salzburg is a 2 day city. Vienna is a great and large European city worthy of 4 days to see the popular sights.

I suggest she fly to Vienna. 3-4 days there, at least. Then Salzburg. No direct flights to Paris, but Eurowings should be inexpensive. Just a cursory Google search should give you/her the top sites in each city. She can then choose which best suits her.

I'll echo David's sentiment.

We were in Germany and Austria this past summer. Salzburg itself is worth 2 days max in terms of the sites within the town. We actually spent four nights there total there but one was dedicated to Bertchesgaden [Konigsee in the am and Rick's recommended Eagle's Nest Tour in pm, was freaking awesome cause it was a clear day], and then explored Salzburg the next two. After that we went on to Halstatt for two nights and then drove to Vienna by way of Melk. We spent 4 days in Vienna and left feeling like that really was the perfect amount of time. We did jam our days full but if you're going by Rick's book, I think we only left a few "1 triangle" things on the table. It just took paying careful attention to when things were opened/closed. We had a car up until drop off on arrival in Vienna.

If she wants to do both cities, I'd tell her to block out a week in total, accounting for a travel day between the two. I'd also go mid April for better weather.