boat travel to Vienna
 

Is there a boat that travels from Prague to Vienna? If so, how do you make a reservation?

I would assume the Vltava meets the Danube? I would do a little more research if I were you...maybe google Danube boat rides or something like that to at least get you started.

I think the Vltava flows northward into the Elba.

The only boat service out of Vienna that I know of is downstream toward Bratislava and Budapest or upstream toward Krems and Linz.

No boat service exists from Prague to Vienna.
You may be confused as some boat tours advertise both cities on tours. The fine print will tell you that take a bus to / from Prague from the boat on the Danube.

Are you sure you aren't thinking of Budapest to Vienna?

http://www.mahartpassnave.hu/

is the website I found for info on this trip, which I am considering in about two weeks.

Andrew