I would like to find a company that gives a river ride from Budapest to Vienna in just one day. Is this possible? We just want to use the boat for transportation.
I get the impression you are looking for one of the major river cruise boats that can take you aboard for just that leg. Don't know if that is possible, but I know a gentleman that might have an answer. I will ask him this evening. The other option is the hydrofoil service but it is bumpy, noisy, expensive and takes nearly 6 hours. Now that I think about it, if the high speed hydrofoil takes 6 hours a river barge is going to take considerably longer. I've been told that the stretch of Danube from Vienna to Budapest isn't particularly attractive except along the Estergom to Szentendre bend (which you can see by boat from Budapest). http://www.mahartpassnave.hu/
The hydrofoil is the only "mass transit" boat between Budapest in Vienna. Note that it only runs in the warm months, April to Sept., departing once a day. Budapest to Vienna takes a bit longer (6.5 hrs.) since you are going upstream.
I asked around to see if anyone knew of the riverboat tours took on day passengers with no luck. If nothing else the schedule would kill you as some (all, many?) of the boats overnight Vienna to Budapest which makes sense as there isnt much to see until you reach the bend. Sorry.