You have serveral options here,
1. you could attempt to add the exe as an additonal file to be scanned,
Instructions on how to do this are here;
However as you indicate this is a dll rather than a exe, this may not be possible, it depends on the scan levels you have set etc.
2. You can create a custom defintion for the dll file,
This should then allow you to track the version of the file and the machines that have it or not. Then allow reporting on it as a vulnerability rather than a piece of inventory data.
Instructions for setting this up are here:
There are some other useful things you can do with the Custom Defintion also, you should find the additional information in the links from the second page,
I hope that this allows you get to the bottom of your issue and allows you to track the devices with the plugin you are looking at.