Major operating system updates are definitely something to proceed with caution with - particularly when they're so new like El Capitan. 


Virtual Drumline is simply a library that loads into Native Instruments Kontakt Player. So whether or not VDL is going to be compatible with your operating system upgrade will depend on whether or not Kontakt is compatible since it is the actual program (or plugin) that runs on the OS.


Upon the release of El Capitan in Fall of 2015, Kontakt is not supported on El Capitan. Native Instruments suggests that users do not upgrade to El Capitan until all the compatibility issues have been worked out with Apple. They've published all their compatibility info here.


Native Instruments: Mac OS X 10.11 Product Compatibility Chart


Also, keep in mind that if you're hosting Kontakt within another application (like Finale, Sibelius, Digital Performer, etc.), it's important that you verify that those programs are also supported on the operating system update. It's not uncommon for software developers to ask their users to pay to upgrade to the latest version of their applications to ensure support with the latest operating systems. So consider that you may have to pay for a third-party software upgrade before you'll be eligible for the latest OS compatibility.


Here are links to Avid (Sibelius) and MakeMusic (Finale) articles regarding El Capitan support.


Avid: OS X 10.11 (El Capitan) support information for Sibelius


Makemusic: OS X 10.11 (El Capitan) support information for Finale


If you have already updated to El Capitan, and have discovered your third party applications are not compatible, we suggest restoring your old system from the backup you made prior to upgrading, then waiting to ensure all your third party software is compatible before upgrading.