For a south-based couple, Sao works as a morning beach stop followed by lunch, a return to the room and an evening around Sunset Town. That room break matters: sand, sun and a late performance make a long continuous day. Someone already spending the night in An Thới or Khem has little reason to insert a northern wildlife excursion just to complete a list. Save that coast for a trip with time to appreciate it.
For a family staying around Bãi Dài or Gành Dầu, a Rạch Vẹm excursion can start after breakfast, continue to the agreed shore or boat stop, and finish with a coastal lunch before returning north. It need not become a full-island tour. From Dương Đông, either choice is possible, but choose the stronger wish: white sand and beach services, or the northern landscape and a seafood outing. Allowing roughly half a day is a planning suggestion for either version, not a travel-time promise; a boat stage or a leisurely lunch can extend it. Keep the afternoon free rather than attaching a timed activity immediately after the return. That is how the day retains the unhurried quality the photographs suggested in the first place.