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There is a "Bratislava Ticket" for β¬16 that covers a return journey from Vienna to Bratislava on any train, plus local buses and trams in Bratislava. You can buy this at the station just before you get on the train. A seat reservation isn't necessary, and I'm not sure if it's even possible.
See http://www.oebb.at/en/angebote-ermaessigungen/euregio-slowakei:
Travel inexpensively to Slovakia with the Bratislava-Ticket (known as Euregio Slovakia). The ticket can be used with all public urban transport trains.
On the printed first day of validity, the Bratislava-Ticket is also valid as a day ticket for public urban transport in Bratislava (until 01:00 a.m. of the following day, or on 1 January until 06:00 a.m. of the following day).
There may be alternative tickets you can get if you just want a single or don't need the local transport. I'm not sure if you need or can get reservations with these.