Before I update my operating system to Yosemite, are there any issues with VDL anyone has encountered?

There should be no issues with VDL or the Kontakt/Kontakt Player. Make certain you are using the latest version of Kontakt, though. Just to be on the safe side.

Depending on whether you use Finale or Sibelius, and which version, there may be some potential issues. Again, updating to the latest versions will help to head off most potential problems.

Actually I've heard some mixed results about Kontakt with Yosemite. Here's what NI has to say about compatibility��_

[url=http://www.native-instruments.com/en/support/knowledge-base/show/3163/mac-os-x-10.10-yosemite-compatibility-news/]http://www.native-instruments.com/en/support/knowledge-base/show/3163/mac-os-x-10.10-yosemite-compatibility-news/[/url]

I'd say proceed at your own risk until NI has posted full compatibility reports.