It really depends on how much time you are spending in hong Kong/ aiwan.
Jap B enceph. is quite unusual in Hong Kong, and hence it is not part of the routine immunisation. It can only be transmitted from pigs (+ few other animals) to human via a mosquito as a victor. There is no human to human transmission. According to th CDC guidelines, you don't really HAVE to get vaccinated unless you are staying near a high risk area for at least 1 month.
Some of the Asian countries give out Jap B vaccines as a part of the immunisation - eg Japan, South Korea, Taiwan. There were several episodes of scattered outbreak back in the 60s ->80s, and the incidence of the disease have dropped quite dramatically since then. There are still sporatic cases but they are mainly in the rural areas of the country.
I think if you won't be staying in the rural area/ high risk area for a prolonged period of time, getting the vaccine is not essential. Ask you paed to see what he/ she thinks.