Goodereader posted an article called "Essential Kobo Vox Apps and Tools", which you can find here
. They include a link to the Sony app, which you can install on your Vox.
My sister has a Sony eReader and I know she installed the app for her Sony books. I haven't checked with her if she's used it to access the store. Since she didn't complain afterwards (I'm usually her go-to person if she has tech problems) I assumed it works fine.