Vistor is not a tour company, meaning that it doesn't run the tours itself. It sells tours run by a bunch of different companies. That doesn't mean there is anything wrong with the tour you have found. I have always wondered, though, whether it would be possible to buy the same tour at a lower price by Googling and locating the website of the company actually running the tour. I have never shopped Viator for tours, so I don't know how their prices compare.
There may be multiple companies running bus tours to the same set of towns, so even if someone else has bought a tour that sounds like yours from Viator in the past, it might possibly have been run by a different company.
There are some hill towns above the Riviera that are very tricky to reach by public transportation, and stringing more than two of them together in a single day can be quite the challenge if you are uskng public transportation, so there is a place for bus tours. However, St.-Paul-de-Vence is known for three things: being incredibly picturesque, being extremely touristy (yes, those two go together), and being the site of the fabulous Galerie Maeght (modern art). If you're interested in the gallery, I'd be concerned about not having enough time to see it if on a bus tour. If you're not into modern art, then there's no concern on that score. I mention SPdV specifically because there is bus service to that town from Nice, and if you want to go to the gallery and explore the town, you probably wouldn't be trying to hit more than one other town that day, anyway. I saw both SPdV and Vence in a single day.
