Train travel between Vienna, Austria & Venice, Italy

https://www.seat61.com/trains-and-routes/vienna-to-venice-by-train.htm

The first company you should avoid is Rail Europe. They are obviously giving you very bad advice.

The company that runs trains between Vienna and Venice is ÖBB, www.oebb.at, and you have two daily day trains and one night train to choose from if you want a direct journey.

Definitely avoid Rail,Europe. They are in the business of selling marked-up tickets on routes that they can sell at the most profit. They do not show all the possible options.

I like to us bahn.de, the German train website, for initial research on possible routes. Then when I want to buy the tickets, I go to the national train website.

https://int.bahn.de/en/

You will see that there is a direct train (no changes) from Vienna to Venice departing at 12:18 and taking just under 8 hours. You can purchase that on the Austrian train site:

https://www.oebb.at/en/

There is also a night train, but I would not recommend that. What I would recommend, were we doing this journey, is to break the journey into 2 segments, with an overnight in between. Innsbruck would be ideal for that, close to halfway. There are direct trains from Vienna to Innsbruck ( under 5 hours) and also from there to Venice (about 5 hours) or you could choose a train with a change at Verona.

Depending on your actual date of travel there may be construction issues with detours in place, so the routes could be different. And if OeBB approves too difficult (I find their website challenging but manageable), the one third-party ticket seller I could consider is Trainline.com. However, as with plane flights, if you use a 3d party to buy tickets, you need to go through that agency if you encounter problems or want to make changes. If you buy directly from the train company, their customer service line will usually help you.