Care to share where you spotted the croc.... I've been to SBWR quite a few times but have never seen any crocs. I was fortunate to spot otters though but didnt have my cam with me that time!
Agree that the Mekong Delta (at least the part I visited) is not worth a 2 days trip especially from a photographic standpoint. The Tonle Sap in Cambodia near Siem Reap is much better in this respect.
I think your gear should suffice. Never heard of the place but checked it out on the net - mountainous region with the biggest waterfall in China. You'd probably end up using more of the short end of the lense. Have a good trip and do post some pics when back!
BTW, which part or parts of China are you going to? That will narrow down the scope and perhaps you can get better advice on what lens to bring along...