If you fly into SGN you'll need to get a visa in advance.
Also, I've noticed a decrease in tourism in the last few weeks in PNH (buffer time between summer and high season?). This time if year is great as the rains decrease and the temps are moderated.
Yesterday I noticed the hyacinth was floating down from the lake to the Mekong. Seems the seasonal flow of the Tonle Bassac has reversed already.