You can NOT leave the transit area of the airport without a Vietnam visa and must have a same-day connection.
VN is a decent carrier...took 3 flights with them last month. Certainly just as good (probably better) than anything flying in the US or Canada.
17MAR08 / 2235 UTC
National U.S.A. (US) /Embarkation U.S.A. (US)
Transit VIETNAM (VN) /Destination CAMBODIA (KH)
ALSO CHECK DESTINATION INFORMATION BELOW
VIETNAM (VN)
Visa not required provided continuing on the same day and
holding confirmed return or onward ticket.
Leaving the airport transit area is not allowed.
If arriving by Vietnam Airlines only, it is possible to obtain
a transit visa on arrival, provided holding letter from
Vietnamese Immigration confirming that a visa will be issued
on arrival. Fee: USD 25.-. Also see: For details, click here