Is it possible to customize the Inventor Vault Add-In (VC 2012)?
Specifically, I would like to capture an event (like lifecycle state change) and fire off a task. This is possible with the client but I can't find any documentation or samples for the Inventor Add-In.
The Vault SDK doesn't provide a way to extend the functionality of the Inventor Add-in itself, but you can capture events for lifecycle state changes using the SDK's web service command events feature. All clients, including the Inventor Add-in, will fire these events and they will get passed to your extension. An example of this can be found in the Restrict Operations sample that comes packaged with the SDK. More information can also be found here: http://justonesandzeros.typepad.com/blog/2011/04/f
Sounds like you might be concerned that some clients might connect to the server when your particular extension isn't running to catch the web service command events. Unfortunately, the server doesn't know what extensions the client has loaded, so no, there isn't a way to prevent this on the server side.
For an extension that is made to restrict an operation on a client: if any clients were missed during deployment of the extension then the restriction would not be enforced. A user would also ptentially be able to override the restriction.
If you override the GetRestrictions event on a web service command, any clients should not be able to override those restrictions, but you are correct that if a client is missed during the deployment of the extension, the restrictions would not affect that client.