A colleague of mine in the UK reminded me about this issue just today. The issue is that you have a nice working Enterprise Vault 8 SP 4 client installation with Virtual Vault, and you upgrade to the client extensions to 9.0 or 9.0.1, and the Virtual Vault node in Outlook disappears.
My colleague wanted to know if I had seen, and it definitely rung bells!
I eventually tracked it down, and it might be something that customers encounter. Here is a reference explaining it visually and linking to the fixes :
It is fixed in EV 9.0.2 client (which of course should be what you’re rolling out to clients now anyway, right?)
It is fixed in a 9.0.1 Cumulative Client hotfix. Unfortunately the one in the technote is not correct (it points to an “old” Cumulative Client), so go for :