1) You wouldn't technically be "upgrading", you'd be "reinstalling". Upgrading is "Vicidial 2.12 build XXXX to Vicidial 2.14 build yyyy" which involves downloading a new version of the Vicidial code, installing that new copy over the old one, and then upgrading the database. But for ViciBOX (not ViciDIAL), you delete the HD, reinstall the OS and everything else (including a new version of Vicidial, of course) and then restore your database and upgrade it to match the newly installed Vicidial code. So there is some "upgrade" involved, but it's really a full reinstall.
2) No. Until they push Viciphone onto a Vicibox install, you have only gained some "cool toys to play with", but not actually gained any features by altering your Vicibox Installer version. The latest version of Vicidial in either installer is still the same "latest" version of Vicidial with the same features.
3) There may be some slight advantages to a later version of the OS, but those will be technical OS-related issues rather than features of Vicidial itself. Such things as security certificate installation. The ability to upgrade to asterisk 13 and "prepare" for viciphone.