Ayahuasca is not one plant. The brew is made by pounding the stem of the Banisteriopsis caapi liana and boiling it for many hours with the leaves of a second, usually chacruna. The vine contributes no visions at all. What it contributes is a group of beta carboline alkaloids that inhibit an enzyme in the human gut, and that inhibition is the only reason the dimethyltryptamine in the leaf survives being swallowed. Taken alone, the leaf does nothing whatever. Taken alone, the vine purges and sedates.
This is the fact that makes ayahuasca remarkable before any question of religion arises. Two unrelated plants, out of the tens of thousands in the Amazon, must be combined in the right way for either to have any effect, and nothing about the appearance, taste or individual action of either suggests the other. Explanations offered for how the combination was found range from very long systematic experiment to the answer the traditions themselves give, which is that the plants said so.
The brew is taken at night, in ceremony, under the direction of someone who has undergone a long apprenticeship built around la dieta, a restrictive regime of food, salt, sugar and sexual abstinence held for weeks or months while learning from one plant at a time. The singing is not accompaniment. The icaros are understood to shape and steer what happens, and a ceremony without them would be considered not a lighter version of the thing but a different and unsafe one.
Sources Plants of the Gods: Their Sacred, Healing, and Hallucinogenic PowersRichard Evans Schultes and Albert Hofmann with Vegetalismo: Shamanism among the Mestizo Population of the Peruvian AmazonLuis Eduardo Luna