There are actually a few tunnels and mountain passes in Austria where you have to pay additional toll, but between Vienna and Salzburg, the regular toll sticker will cover everything.
Rental cars in Austria will at least from the larger chains usually already come with toll paid. I see now that Sixt guarantees that cars rented in Austria have toll paid in advance, other rental companies are more vague and with the cheaper budget rental companies, I would not be quite sure what to expect. Be aware though, that the toll can now also be paid and connected to the car's registration number without having the physical sticker attached to the wind shield. If the rental car doesn't have a sticker, the autobahn toll may still have been paid.
You can use Asfinag Evidenz to check if your car already has a valid digital toll sticker. Enter the registration by selecting Austria and the registration number of the car in format W-12345A.