Food on the train from Vienna to Budapest?

Impossible to answer without knowing what kind of train you've bought tickets to.

But if it's a Railjet, then yes. Here is the current menu: https://www.oebb.at/dam/jcr:17e4b6bd-f5d0-4fe1-8e9a-a47a7abae5dd/folder-don-bistrokarte.pdf

I've always found a good selection of train picnic offerings at European train stations.

It looks as if you are on the EC 143 which does convey a restaurant car as the train is on a much longer 12 hour journey to Cluj Napoca in Romania, so you should be able to get lunch there, depending on the opening hours of the restaurant car. It is always possible they may not open until after Budapest, so may be best to take a picnic with you just in case.

Vienna train station has lots of places to get prepared food for the train including a nice food court. There are also two large supermarkets on the lower level and two smaller versions of the same markets on the main level. The smaller markets have more prepared foods. I think Billa is the better of the two for prepared foods. (The other is Interspar.) For more selections, go one floor down to the bigger Billa. It had a separate area just for takeaway foods including a salad bar.

Thank you so much for all your information, it was so very helpful. FYI we are traveling on Regiojet.

Regiojet does not a have a restaurant car, but they have a trolley service onboard.

In general, this is easy question to answer by visiting the company's website. Also, if you had bought the ticket direct from the company operating the train instead of using a 3rd party, the information would have been there when you bought the tickets.

You might do better to hold on until you get to the Deli train station in Budapest and eat in the Castle Hill district. Not a lot to eat there, but the atmosphere is nice and it's a good way to start tge visit. And you might as well take the break because its a pretty fair trip to Pest. Unless you plan on getting off at Kelenföld in which case I would just take the metro to town (Pest).