Hello Fern,
Interesting boats! Thanks for posting. I'm not exactly sure...but if I had to guess I would think perhaps a very simple working knife like:
http://www.vikingsword.com/vb/showthread.php?t=5163 which is fairly common style from Burma to at least the hills of Vietnam ...that being said I'm not sure, what the Vietnamese coastal proper knives would look like perhaps though like a short Chinese DaDao/ Vietnamese Douk Douk...ie something with a full tang.
That being said these are the types of knives though used for splitting rattan or bamboo or as a draw knife for wood shaving/ cutting in and around Thailand:
http://www.vikingsword.com/vb/showth...attan+splitter
I would not be surprised though if what is used really is not at all that specialized, but just a general work knife that they use for everything!