On the Kobo Vox, what you do once you have the book on your PC is to get it into Calibre.
Then plug the Kobo into the pc and get calibre to send it to the device. Once it has apparently done that (it seems to give no visible indication that it has done anything) unplug the Kobo.
Then, using the kobo and with the library open, choose the menu button and select import. If the menu does not contain Import, you are not in the library.
The book will be then readable.
One of the things you will notice during this process is that the Kobo messages seem to think that you have the book on an SD card which it offers the option to delete. I think that what happens is that Caliber puts the book into a holding area and the Kobo Import copies into the correct place in the Kobo for ereading.
If you don't choose to delete it, that boook will be read in over and over again every time you get a new book, and in the end the hilding area will be full so you will have to delete them.