Saw you note and just now tried this on a setup here (VS2008, Vista Ult. 32bit, 04CTP). I created a new sln/project, and ORM file. Went straight to context menu and opened Extension Manger. Clicked Relational View, and saw just what you stated - a vanishing act! Tried other EM slections - same thing. Tried after entering some Fact Types to the model - same thing.
Another system here (VS2005, WS2008 32 bit, 04CTP), has EM woring fine! My understanding is that the engine of Vista is much like that of Windows Server 2008, so if that's the difference, it's subtle. On the other hand, the VS2005 and VS2008 versions of CTP are (I believe), even closer.
Following a lead from Matt C., I made sure the GAC was clear of neumont artifacts, after removing old CTP, OPliX, DSL Tools; and before installing new components. I did not install new DSL Tools on Vista/VS2008 system, as I understand that VS2008 includes these - there were DSL Tools on that system from an earlier CTP install (possible link?).
Both systems installed and run with full admin authority.
Don't see any options in VS ORM Designer that would affect the EM.
I only looked at the Relational View in VS2005 system because I was curious. I haven't gotten to anything yet where it's needed. As the View is for the whole model, it's too much info for all but simple models. I think I'd opt to shove the generated DDL it into SQL Server and using its tools to manage the Relational (logical) View. Not the same, but may tide you over, until there's a fix.
Glad you reported the bug.