If you land at 11:00 AM, you’ll need at least ~20 minutes to get out of the airport (assuming no checked luggage).
From there, it’s about 1+ hour to Halong, so you’re realistically arriving around 12:15-12:20.
By that time, almost all group cruises have already departed.
it’s quite risky if you're trying to join a shared cruise

You might still have an option if you go for a private charter.
Both day cruise and overnight cruise can sometimes arrange flexible pickup times, but obviously it’s more expensive.

You can arrange a private car waiting at the airport, go straight to the harbor, and then take a small private canoe to catch up with your cruise after it has already departed.

Costs around $80–100 from what I’ve heard.

You need to coordinate this in advance with the cruise operator so they can arrange the transfer boat

If you want a safer option, there’s an earlier VietJet flight that lands around 7:30 AM.
Much more relaxed timing and you won’t have to stress about missing the cruise.
Downside is… very early wake up 😅

Halong Bay is something you don’t want to start stressed or rushed.
Better to arrive the day before and enjoy it properly

thanks for your recommendations