You mean adding it to the product xml?
Code:
<dependencies>
<dependency dependencytype="vbulletin" minversion="3.7.0" maxversion="" />
</dependencies>
You can easily set this up in the adminCP by adding dependencies in your product,
then export the xml to view how its coded.
http://www.vbulletin.com/docs/html/product_dependency
I use a dummy product just to test things and study how they should be coded.