Can I enter Croatia from Austria with a single-entry Schengen visa by bus, and then return to Vienna by bus ?
Croatia is outside the Schengen area. They do accept dual and multiple entry Schengen Visas but not single entry ones.
As stated in Timatic, the database used by airlines
Visa required, except for passengers with a double or multiple entry C visa issued by a Schengen Member State valid for all Schengen Member States and valid for the period of intended stay. They are visa exempt for a maximum stay of 90 days
So only double and multiple visas are valid, and it does not matter if you travel by bus, train, ship or plane. Meaning, you need a separate Croatian visa