Tet is all about starting out the new year correctly. That means cleaning and decorating your house, paying off your debts, paying respects to your family and friends and most importantly -- eating well. In Vietnamese, you can say "an Tet" to literally denote eating Tet. Tet Nguyen Dan (translated as the "festival (or feast) of the first day") is the central holiday in Viet traditions. It's the only time during the year when the entire country is relaxing and resting. While many ideas are borrowed from the Chinese (e.g., firecrackers, lion dances, red money envelopes), the food part is pure Viet!
What's eaten differs from region to region in Vietnam because of differences in weather (thus limiting the ingredients on hand) and taste preferences. My parents are northerners but us kids were all born in the south so what we have enjoyed food-wise during Tet spans the entire country! Note that these are foods that will keep well for days. They're meant to be so because during Tet, you're suppose to be out having fun not slaving away in the kitchen!
If I had to limit myself to just a few items for Tet, I'd get my hands on the following:
Banh Chung - A square shaped rice package of sticky rice, mung bean puree and seasoned pork. It's wrapped in banana leaf and when you buy it at Vietnamese markets, it should also be wrapped in plastic and perhaps, tied with a little red ribbon for good luck. Banh chung looks like a small adobe brick. My mother makes hers but the 6-8 hour boiling process that cooks the packages is enough to deter many Vietnamese people to conveniently buy it. When you get your hands on one, you can eat it just as is if it's somewhat soft still -- an indication that it's fresh. Poke at it with your finger to get a sense of freshness. Then peel back the layers (the rice should be tinged green from the banana and bamboo leaves) and cut it into wedges. You can use dental floss to cut it nicely if you want. The wedges ensure that each piece gets an equal portion of mung bean and pork. This may sound odd but I like to dip banh chung in a little sugar. The photo below is of a banh chung from Vietnam. Look at the detailed wrapping!
If your banh chung is hard and old or you have leftovers, you can fry it to a delicious crispiness. Cut it into chunks and fry it up into a crispy pancake and again, enjoy it with a little sugar. Use a nonstick frying pan, a little oil and moderate heat to make things easy on yourself. During the frying process, the banh chung will soften, giving you the opportune moment to use a wooden spatula to flatten it out and mesh it with the other chunks into a unified pancake. Flipping the banh chung pancake is a little tricky so use a plate to invert it out or if you're daring, flip it in the pan with a flick of the wrist. Sticky rice is pretty forgiving. For more details, peruse this page on banh chung and banh tet.
A caramel sauce dish - These dishes, called kho in Vietnamese, are terrific must haves for Tet. In the south, it's fatty pork leg, eggs and coconut water. Stewed pork riblets are excellent and so is a kho made with beef drop flank and ginger. These dishes can be prepared days ahead, which explains why kho (salty simmered/stewed meat dish) dishes are present at traditional Tet celebrations. If meat isn't your thing, then cook shrimp or fish in caramel sauce.
Viet cold cuts - Here's another convenient food for the holiday. These cold cuts can be purchased at Vietnamese markets or delis. If available, get some gio thu (headcheese), gio bo (silky beef and dill sausage) and cha mo (steamed pate of pork and lardons that's fried to a crisp in the end). Gio lua (basic silky sausage) is also good and to eat, you can buy some banh day, steamed sticky rice flour discs on banana leaves; banh day look like hockey pucks and are sold in Viet markets and delis too. Put a thickish slice of gio lua or cha mo in between the two banh day and dip it into a little fish sauce (thin it out if it's too salt for you).
Pickles - Those cold cuts need and accompaniment like a pickled shallot or leek -- both are often sold in plastic bins in the produce section of a Viet market. Small white pickled leeks are also sold in small cans at Chinese markets. These members of the onion family are meant to help facilitate digestion of all the rich meats eaten during Tet.
Nuts and candied fruits - At the markets you'll find a wild assortment of brightly colored candied treats such as coconut, melon rind, and lotus seed. Also, watermelon seeds are also eaten during this holiday. Pick up one of these assortments for a fun nibble. They're typically a bit insipid I have to admit. But price dictates quality with regard to Asian food products, so spend a little and you'll be well rewarded!