just bear in mind that there are currently access questions at Tam Dao, for certain (see my recent thread) and apparently Ta Nung also (a locked gate that may be passable for a fee if you're lucky, plus overgrown trails and ongoing construction* - thankfully there are other alternative sites in that area, although possibly not as good for Crocias).
the Dalat plateau has more of the Vietnamese endemics than anywhere else, and a large percentage of the near-endemics and putative endemics so has to be the absolute must visit area if you are limited to one.
see here
http://www.vietnambirding.com/bird-specilities.aspx
6 of the 12 listed endemics are found only in the Dalat region (4 others in various locations in the Annamite mountains and 2 in the far north near Sapa), and there is also Dalat Shrike-Babbler which is split by the IOC.
bear in mind however that there are no good bird guides based in Dalat so the cost of a day out with one will likely be prohibitively expensive as it will need to cover their travel and accommodation from HCMC or Hanoi....
James
* for Andy's benefit this has all happened since 2002

B