Like other Asian countries, Vietnam has some religious holidays that Buddhists observe. Typically, New Year day is always the biggest day for celebration. Note that these holidays occur in lunar months. Here are some of them:

1/1 New Year Day. Commemoration of Maitreya Buddha
1/15 The first full moon day of the year
1/19 Commemoration of Quan Yin Boddhisattva
4/8 Sakyamuni Buddha's birthday
4/28 Commemoration of Medicine Buddha
7/15 The second full moon day of the year
It's also the Ulambana day
7/30 Commemoration of Ksitigarbha Bodhisattva
8/15 Full Moon Festival
It's also the Ulambana day
11/17 Commemoration of Amitabha Buddha
12/85 Buddha's Enlightement

Traditionally, people attend temples to do incense offerings or fruit offerings and chant on these religious days. Some even eat only vegetarian foods. It's always crowded at the temples on the new year day and Ulumbana day as I recall. Normally, people pray for the best on the new year day. After the offerings, they are offered a small gift as a good luck to bring home. On the Ulumbana day, people gather at the temples to pray for their ancestors and make good deeds such as donation, free animals,etc... Meanwhile, an important ceremony occurs: Robe offering to the sangha. It's quite joyful to participate in these events of the year.