Its good that some elements of Van Buren will become canon now. (Ceasar's Legion, Hoover Dam etc.)
But remember that there may be changes. Just because they include Ceasar's Legion, this does not mean that it will be exactly as described in the VB design documents.
There may be changes to their historic timeline ,setup, motivations and characters etc. to accomodate the storyline ideas for New Vegas.
The content of Van Buren is just a draft and only what of it will end up in future installments of Fallout will be set in stone Fallout Bible canon from that point on.
However it seems to me that Obsidian and especially Josh Sawyer and Chris Avellone understand the respect the Fallout fans have for Van Buren and the Fallout Bible.
While saluaging some interesting elements of Van Buren and keeping them as much as intended to be in Van Buren, they will make sure that nothing of what they put into New Vegas will really contradict anything from the timeline and the locations of Van Buren.
If you ask me for my personal view on things, The Cheyenne Mountains Facilities are that nuke overkilled area of huge radioactive craters described in the Van Buren design documents and not that Vault 0 from Fallout Tactics.