OK. I will take your word for it, because I have not used Vault yet. We are supposed to be getting into it in the near future though. The term 'Open' can be a bit misleading, because it is possible for a document to be open, but still not be visibly showing on your screen, and with no document tab. We can very easily open documents invisibly by code, when we do not need to actually see certain changes being made to it by our code, to reduce processing time and use fewer system resources.
There are also a couple other terms in this general area that you may need to be familiar with, if you are not already. "Initialize" is one of those terms. That is when a file that Inventor is seeking for some reason first gets found and a portion of its data read into Inventor's memory, for the purpose of following file references, such as when some other document gets opened in Inventor, and it is referencing that file. This is similar to a 'partial' open, and is a step that happens before a Document is fully opened, and that document may never get fully opened, depending on the situation and events to follow. Another term is 'Activate', which essentially means making a Document the 'active' document, which visibly shows it on your screen, ready for direct edits by user interactions. It is possible for there to many documents open, both visibly (with document tabs), and invisibly (no document tabs), but only one can truly be the 'active' document at any one time.
With that in mind, we can 'listen' for those very specific events to happen, and react to them. But when relying solely on the simplistic iLogic Event Triggers dialog settings for reacting to events, we are not given that many options for events to use, and not given much control over the timing of our code based reactions. In order to get access to more events, and more control over those events, we need to utilize the Inventor API's resources, instead of uniquely iLogic resources, which generally requires more advanced coding, and is more complex to manage. See the following online help links to some related Inventor API events.
ApplicationEvents.OnInitializeDocument
ApplicationEvents.OnOpenDocument
ApplicationEvents.OnActivateDocument
ApplicationEvents.OnNewDocument
ApplicationEvents.OnDeactivateDocument
ApplicationEvents.OnCloseDocument
ApplicationEvents.OnTerminateDocument
...just to mention a few that are available from the ApplicationEvents object, which itself is just one of many such Inventor API objects with events associated with them. Handling Events like these are generally better managed by Inventor Add-Ins, rather than with normal iLogic rules, but we are able to use them with our rules also, if the rules are well enough written and efficient enough. But if the code is not written that well, or is not that efficient, it can negatively effect Inventor's performance, depending on the event(s), and what is being done in response to the event(s).
Edit: It may also help to be aware that...when we open an assembly or a drawing type document, those document types usually always 'reference' other documents & files, because they usually contain representations of those other documents within them. So, when we open them, all those other documents that they reference also get 'initialized' in the background, invisibly, with no document tabs. You can notice this taking place if you have Inventor's status bar visibly showing while opening those types of documents, and watch the two numbers on the status bar's right side. The number on the left is 'total occurrences'. The number on the right is 'open documents'. When you open a large assembly, you will see those numbers grow, indicating the effects of opening that assembly, and all the occurrences it contains, and all the documents it had to 'load' into Inventor's memory. The use of ModelStates can effect those numbers, due to suppression, which unloads some of that type of data from Inventor's session memory, for performance purposes.
Wesley Crihfield

(Not an Autodesk Employee)