Softmon does not work in an Citrix Environment, so from a usage standpoint you will have to use Citrix to track license. My understanding of virtual license management is that if app X has a control group assigned to it say of 400 people than you have just licensed application X for 400. Does not mater how many users actually use the application if 400 have rights to the app you are purchasing 400 licenses.
Zman is correct that SLM is not what you need here. regarding the licensing side though, that is really down to the vendor of the app itself. Find out if it is tied to number of users that actually use it, number that use it concurrently, or if it is simply down to the number of users that could theoretically reach it.
Licensing in a Citrix/Virtual environment can be tricky so find out what exactly it is you are supposed to be measuring against and then see if Citrix functionality can do what you need through modules such as Edgesight.
The One-Stop Shop for LANDesk Enhancements