TET is a very big deal in Vietnam. So much so actually that the entire country basically grinds to a halt for a week or so.
The uninitiated make the mistake every year for not preparing properly and they learn to regret this decision because almost all businesses are closed and those that remain open charge around 20% more for food and drinks as it is traditional to pay staff members more that decide to work during the holiday.
src
I will admit that I don't know a great deal about what actually goes on inside of Vietnamese households during TET but I do know that it is the celebration of the Lunar New Year and it is of considerably greater importance than what most western nations consider to be the New Year.
Also, TET is not on the same day every year but it normally falls in January or February.
This is a time that Vietnamese will return to their hometowns and celebrate with family. It is the most important holiday of the year as far as I am aware and for the expat population this can end up being a period where it can become difficult to find places to eat. In even more worse situations, people who neglect to get money out of the ATM machines (as is the only option for cash for most of us since we can't have bank accounts and there aren't many currency exchanges) you can find yourself in a situation where you are at least temporarily "broke."
src
aint gonna be nothin in any of these!
Of course unless you are a complete recluse, you can borrow money off your friends and you'll need to, because it was my experience 2 years ago when I was "green" that I was completely unaware that the ATM machines were not going to be serviced or refilled at all during this time period.
It is quite normal for ATM machines to run out of cash on the weekends anyway, but now that we are talking about a 10 days stretch (it varies depending on where you are) you can count on the ATM machines being cashless for at least 7 of those days.
src
Some friends of mine visited Vietnam during Tet last year and they were expecting parades, fireworks, celebrations in the streets and general public merriment the likes of which we would see in the west during "regular" New Year.
Instead they encountered empty streets and mostly shuttered up businesses. I remember this time well and how we ended up going to the same bar / restaurant for basically every meal since virtually all other establishments were closed at that time. This was a touristy place to begin with and one could successfully argue that their prices were already a rip off. During Tet, it was 20% more expensive than it normally would be and this at first was annoying but then we had no choice but to laugh it off since this price increase is kind of fair when you consider that the staff members at these places are actually foregoing a very important time of year to be spending with family back wherever their homes are.
Not that it really matters this year since people are, as far as I know, forbidden from traveling into Vietnam unless under very certain circumstances, but if in the future you are expecting Tet to be a time of parties and festivals you need to rethink this because you are not going to encounter any of that.
For me, I plan on stocking up on steaks and vegetables and eating a bunch of meat and drinking some wine while watching a ton of movies during these 10 days. Any attempts to venture out for some nightlife will almost certainly result in failure!