The ACSM file is a "ticket" allowing you to download the DRM'd book with Adobe Digital Editions. You need Adobe Digital Editions installed and authorized with an Adobe ID (not technically required, but recommended), ADE then uses the ACSM ticket as an authorization when talking with the DRM servers in order to download a copy of the book encrypted to you're Adobe ID.
If wanting to read it on your Vox another option might be to download the EBSCO eBooks app and access it this way, provided the app is compatible with that old of an Android version. (not sure about the app, I just see it mentioned in your screen shot).
EDIT: Here's the MR wiki page on ACSM, which may explain better...
https://wiki.mobileread.com/wiki/Acsm
Note: If you think you might want to remove DRM from purchased books (ePub. PDF) with Adobe DRM in the future it's recommended that you stick with ADE 2.0.1 and not one of the newer version.