Many thanks to Roku user AvsGunnar for telling us all that these classical channels are available on the free Live channel on the Plex Channel. Add Plex to your Roku channel lineup the usual way, then go in and use the search function to find Vivaldi and Qwest Classical individually. You don't need to go to Live first, just use the search function magnifying glass by clicking left to the first appearing left pane. I imagine the other Qwest channels are there also, but haven't looked for them. And guess what, the Vivaldi listings are even accurate! Mon Dieu! Beaucoup unbelievable!
And guess what? There are two more classical music channels nearby, Stingray Classica and Stingray Cmusic! Another Mon Dieu!
Thanks for the update.
If you're looking for classical music on your Roku you may also be interested to know that the Detroit Symphony Orchestra has its own Roku channel. Search the channel store for 'DSO'.