I dont believe HK has a law limiting vacation carryover. Its been some time since I looked at the Employee Ordinance - but I'm doing some research...
However, most companies limit the days you can carry over, as the cash payout becomes very expensive as your salary increases. If your company has NOT stipulated "use it or lose it", well then they owe it to you, I guess... I've known companies to payout 30-40 days vacation time when a person leaves, if it is owing. Sometimes people just cannot take leave because of their work load (!) - when this happens (in companies that limit carryovers) it should be documented why they could not take their leave, and then get manager approval for the carry over.
If this is an issue, don't take our word for it - get legal advice. I would recommend talking to Siobhan Mckeating of B&M. She's good.
found this on the net, doesnt really address the issue, but doesnt limit the carryover either.
http://www.careertimes.com.hk/englis.../legal0419.asp
HK's laws are really vague sometimes.