As a Vietnamese national, on a New Zealand student visa, I'm transiting via Hong Kong on my flight home for the holidays. Will I need a transit visa, flying with Cathay?
According to Timatic, the database used by airlines for visa information:
TWOV (Transit Without Visa): Visa required, except for Those who are in direct (Air to Air or Air to Sea) transit if holding confirmed onward bookings, continuing their journey within 48 hours and not leaving the transit lounge.
If you can meet these conditions and remain airside at Hong Kong airport (without passing immigration), this says you can transit without a visa. You would need a visa to actually enter Hong Kong, such as to see the city or stay in a hotel if you have a long layover.
This can also be seen on the website of the Hong Kong Immigration Department, where Vietnam is listed as Visa Required, but not with the "*" symbol that says a visa is required for "those who are in transit and remain on the airside." Wikipedia lists Vietnam on the list of countries whose nationals "are not required to have a visa to transit airside if they remain within the airport transit area."