New Years Eve Festivities

Zoo where? Hanoi or Saigon?
Again which city for hotel Majestic?

Saigon. Should have mentioned that in the OP

The fireworks display will be held at Thu Thiem tunnel (District 1) and Dam Sen Park (District 11), on December 31 from 00:00-00:15. You can find some rooftop bars or restaurants to see the fireworks (such as Majestic Hotel, Chill Sky Bar, Shri Restaurant & Lounge, EON 51  Fine Dining  Cafe  Heli Bar, One Plus Beer Club..). Majestic Hotel is quite expensive, about 1.500.000-3.500.000VND for buffet,  I recommend you to go to Chill Sky Bar, this is the best place to watch the fireworks, the price from 150.000 VND-400.000 VND for drinks and you should reserve a table. Or you can take to the streets, participate in the countdown event on Nguyen Hue street,  and then walk to the river beside Vo Van Kiet street to watch the fireworks.

Best place to see and enjoy the fireworks is definitely on a cruise boat on the Saigon River. I did it last year on "Lady Hau" . 4/5 hours cruise on this beautiful wooden boat including a huge buffet (food was delicious) for a ridiculous price.

It would nice if there are more details of your experiences.

I don't know about zoo, but you can watch over fireworks very lively at/near the Majestic hotel. But main streets in Q1 near the Bitexco building, where the fireworks is took place, are closed and packed with so many people, so you have to go there early to get a good view.

Do you visit Vietnam in Lunar New year eve ?
Saigon also has  many nice events I can suggest if you are interested.

Thien Trang's suggestion sounds great. You should also visit Cho Lon market to see how Vietnamese go shopping and prepare for Tet. Well, actually, just walking down town and you should see a new Sai Gon with beautiful decoration. Many people say that Tet in Sai Gon is boring 'cause everyone has gone to their hometown. Well, to me Tet is a great time to explore Sai Gon. The only time in year that you can see Sai Gon is so empty, ride with full speed and feel like you're the only person in the world. Feels like the city suddenly got "lighter" and fresher. Such a nice feeling to me You can take wonderful photos, too, with or without people (If it's not Tet then I guess you have no choice but photos With people). Well, whatever you're looking for, quiet or noise, Tet in Sai Gon has it all. Have a great Lunar New Year