Relationship field lookup in an EE plugin?

I’m in the unfortunate position of needing to fix a third-party plugin that was created under contract; the original developers are not available to fix it.

From within the plugin, I’ll need to look up field data from a relationships field. e.g.: “shirts” has a relationship field that points to the “vendors” channel, which contains a field “email”, so in php using the EE classes or raw SQL, I’d need to look up:

shirts->vendors->email

After a bit of Google’ing, I didn’t find anything documenting the internal structure of relationship fields, or how this might be accomplished.

Any help would be appreciated!