Organized tours offered by Vietnamese travel agencies last from one to four days. I consider the one day tour too short. You won't have time to see much. Two or three days are my recommendations. If you have more time, a fourth day may be added, but I do not consider it necessary.
In Saigon, many travel agencies (and hotels) share similar names. Make sure you book with one of the big agencies. They will offer you a better service. The Mekong Delta tours consist of transportation, accommodation, meals, boat tours and a guide. Drinks and dinner are usually not included. Tour meals on the tour I took were not very good. I recommend having a light lunch (the one included on the tour), and going to a good restaurant to have dinner. In local restaurants the quality of the food is very high, and prices are low.
By the way, it is very normal that you have to change buses several times during your tour. Customers of several agencies may share a bus, if there is not enough people in one group. Do not get upset. It is the way things go in Vietnam.
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