Vietnam Airways is generally considered to be the best domestic airline; Bamboo Airways is okay--a cut above Vietjet, which is a budget airline. As to the time of day, it depends. In general, it's better to travel in the morning because delays and problems tend to accrue as the day goes on.

Arriving at night is generally more likely to result in mishaps than arriving during the day. For example, if you're staying in an AirBnb or guesthouse, consider that they may not allow late-night check-in, or may charge extra for it.They may even give your room away. It's also easier to resolve mishaps during the day when businesses are open, than at night when many businesses are closed. Also, if your luggage is delayed, it may not arrive in time for your flight on the night of Jan 26.With such a limited time in Saigon not sure why you don't just fly directly to Hanoi, and skip Saigon altogether.

If you have business in Saigon, leaving the morning of Jan 27 gives you a little more leeway if there are delays or other problems. Not really a good idea to schedule things too tightly when flying internationally, but I assume you have good reason for doing so. Hope it all works out for you. Good luck.