Hi Matthew,
You'll need an updated version of the 3rd party DLL built with VS2013. The VC110 runtimes are not valid for a Windows 8.1 app, and there is no way to update a DLL short of rebuilding it.
If your vendor cannot provide an updated DLL you'll have to stay with an 8.0 app.
We are aware that this is a problem area but don't have a better solution.
--Rob