Just to close the loop on this question. NCL did have on board a Saigon on your own excursion. It was $79 pp. and dropped you off at Ben Thanh Market in District 1; which is very centrally located. The streets and malls were decked out for Lunar New Year and so I did a Mani/Pedi and walked around, shopped and had a meal. And was quite amazed at the building boom that is going on in HCMC/Saigon over the last two years. I'm sure I won't recognize the place if I go back in two years time.
Thanks to all for the info and advice; I could have saved myself the angst if NCL had that tour available at the outset; however I guess they need to feed people to the other options. The port is very industrial so a transfer is a must to get into the city.