Not justified. Sounds like your particular risk is way too small. Many of us who have lived for long periods of time in Asia where this disease is endemic do not get this vaccination. It's quite rare to find cases in expats who've lived for years in SE Asia.
1) This is an uncommon illness in travelers, and particularly the extreme versions of symptoms.
2) Your visit is short, and in winter. Hanoi and Halong will be cool and mosquitoes not so plentiful.
3) Presumably you will be staying in some sort of hotel (or on boat at Halong) in enclosure, not near rice paddies in primitive huts.
If you were going to be a Peace Corps volunteer living in rice farming (and pig-raising) villages for extended periods of time, I might advise differently. But this money is better prioritized for decent travel medical insurance, and other immunizations you're more likely to encounter, like Hep A.