Yes, we want to go to the Maldives as well, but not for this trip. (Also, the beach and water really doesn't have to be Maldives quality, we're just looking for a nice resort.)
Sri Lanka is far from Singapore as well, and from what I've heard there are no nice beaches.
As for Con Dao, the only true luxury hotel is probably the Six Senses? And the rooms don't look that great...
Why were you unimpressed with the Andaman / Datai? I've read so many good things, especially about the Datai.
Our stay at the St. Regis Langkawi was one of the most beautiful stays ever, despite the artificial beach and not-so-great water.