The best time to travel to the Mekong Delta is during the dry season. The dry season is basically between October/ November to April/May.  You can of course visit in the rainy season too which is between May and October, just be prepared to put up with heavy showers especially in the afternoon. These usually only last an hour or two so are not particularly unpleasant.
The Mekong Delta is an extensive maze of rivers, swamps and islands. This is where you will find floating markets, pagodas and villages.