Zephyr has a searchable help section. This includes demonstration videos and a glossary of terms.
Accessing Zephyr Help - log in to Zephyr, click on Help (top right) and select Zephyr help.
The definition of vendor, and many other terms used in Zephyr, is available in the glossary (see above).
Vendor is defined as:
The entity that is offering the Target company for sale, etc. For example, when Company A buys 10 per cent of Company B from Company C,
This is the advice from the company that provides the database:
See you referencing guide for further advice.