Why not make your Halong Bay trip 2D 1N with one night sleep aboard a junk boat. Hanoi to Halong Bay will take about 3+ hrs and it will take lots of your time. With a tight schedule you might not be able to explore all Halong Bay has to offer - stalactite/stalagmite cave, titop beach plus 422 step climb to view halong bay, kayaking amid limestone island, etc. But if it is only cruising along the islands plus a visit to the cave, your day might be enough.