TFS - how to track when a developer has started working on a bug

[OliverU]

We're using TFS for source-management, builds, and bug tracking. We've got a
mixure of in-house analysts and developers, and external (vendor) developers.

When a bug is marked as Active and assigned to the vendor's coordinator,
we'd like to know when an external vendor actually starts working on a bug -
I'm not looking for an automated solution - the vendor's coordinator could
manually update a value on the bug log.

Have others run into this? We don't keep track of the external developers'
names, so all bugs that are assigned to external developers get the name of
their coordinator. Would you suggest a new check-box? Other ideas?

thanks