Mangrove forests along the Mekong delta estuaries are usually observed to degrade together with the increasing extension of fish farms. In this limited-width condition, the formation of coherent structures, their interactions with mangroves, the role of the width of the mangrove forest, and their effects on the exchange processes between the open channel and the adjacent floodplain are still not well examined. In order to obtain more insight, a unique laboratory experiment of a vegetated compound channel mimicking estuarine mangroves has been conducted. The results show that in a compound vegetated channel with a very gentle transverse slope, the vegetated shear layer dynamics resembled that associated with vegetation rather than that associated with a depth differential. Furthermore, the flow field under the effect of large horizontal coherent structures (LHCSs) shows a spatially and temporally cycloid motion with associated flow events, namely, sweep, ejection, stagnant, and reverse flows. It is also suggested that the coherent structures can have an influence on a broader area than the vegetation area into which the eddy structures can penetrate. In terms of the exchange processes, the momentum transfer and the intensity of transverse fluctuations induced by the LHCSs can be related to this phenomenon. Consequently, decreasing the mangrove width can significantly affect the pattern of the LHCSs, disturb the transverse exchange processes induced by these structures, and thereby changing the shear layer, creating unfavorable conditions for sedimentation inside forests and for river bank stability.