A prepositional phrase is a grammatical structure consisting of a preposition followed by a noun phrase. An adverbial complement is a grammatical function. Adverbial complements may be realized through prepositional phrases or other adverbials.
I put the book down.
I put the book on the table.
I put the book down on the table.
There are verbs requiring complements without which the sentence they are part of would be malformed. In your example, the verb latch is such a verb. The addition of the adverbial particle onto in conjuction with the verb latch gives it an entirely different meaning than if latch were used in a sentence such as Make sure to latch the gate.
Therefore, I would argue that the verbal structure in your example is latch onto and that this structure is followed by the noun phrase a reason. In terms of function, I would say that latch onto is a verb and a reason is a direct object.