See the Header directive in the mod_headers documentation.
This instructs the web server to append the new value of the Vary header to any previous value of the Vary header (separating the new value from the old one by a comma) or to create a new value for the Vary header. The new value to be created or appended will be user-agent. This header will only be created/appended if the dont-vary environment variable is undefined within the environment of the executing apache service.
To summarize, if the dont-vary environment variable does not exist, the server will issue out something like this:
Vary: ...,user-agent
