There are basically a few scenarios depending on timing.

If the ban comes before you even leave (from Hanoi, Hai Phong, Ninh Binh etc.), cruises are usually canceled completely. In that case, you’ll either get a full refund or be offered to reschedule

If you're already on the way or have just arrived in Halong and the ban is announced, things get a bit different. You might still be charged for the transfer, and only get a partial refund for the cruise itself.

If the cruise has already departed and then a storm warning comes in, boats are usually called back early before evening. In that case, the trip is cut short and you’ll typically receive a partial refund.

One important thing to know once a storm hits and cruises are suspended, it usually takes 2–3 days before operations return to normal. So it can affect your whole schedule

A good backup plan is to stay in Halong for a night or switch to another destination. Ninh Binh is a popular alternative, and interestingly, the weather there can still be okay even when Halong Bay is closed

Avoid tight schedules, and try to leave buffer days in case of weather disruptions

Thanks everyone, really helpful to know what to expect!