Then you nested it.
Vegas will then render a proxy file of the nested project and this is what you'll see on the timeline. Search the online help for "nest" for more info. This is nothing new in V9, V8 does the same thing.
If you want to edit a project you need to go File>Open.
You may already know this, but when you open the project in V9 and save it you can't go back and reopen the project in v8.1 so you may want to make a copy of the v8 project before you save in v9.