How many days should I dedicate to Vietnam?

hi - these open tour bus tickets ... im guessing they can easily be obtained once i get to Ho Chi Mihn?? .... can i purchase one at the airport??

Sinh Cafe, the best known of the open-tour lines:
www.sinhcafe.com/english_info/about-us.htm

I doubt if they're in the airports, but easily found in Saigon's Pham Ngu Lao or Hanoi's Old Quarter. Be sure you get the authentic one (copycats all over Vietnam).

The new URL for Pacific Air is www.jetstar.com (it was recently purchased by the Aussies)

I am arriving in HCMC in 12 days and heading to Mu Nie with a friend. Is it possible to book the open tour from Mu Nie? I looked at the web-site but didn't see any office there...

Yes you can book the open tour in Mui Nie...there are several tour offices there which you can book with, just shop around for the best price.