We were concerned about Tet as from what we had heard everything closes down and prices for everything go through the roof.
We were staying at Otres Beach in Sihanoukville, Cambodia over Tet and didn't find that the accommodation prices were much different than usual, but we did find that many places were fully booked. That said there is always somewhere with a free room if you are prepared to look.
The beaches and town in Serendipity did get a lot busier with the locals during 22nd January to the 25th January as a lot of people from Vietnam head to the beaches of Cambodia during Tet.
From what i saw none of the shops/bars/restaurants in Serendipity or Otres closed early or put their prices up.