Does Papias’ vine statement impact on his reliability? Not really, no.
Papias tells us his method in his prologue:
But I shall not be unwilling to put down, along with my interpretations, whatsoever instructions I received with care at any time from the elders, and stored up with care in my memory, assuring you at the same time of their truth. For I did not, like the multitude, take pleasure in those who spoke much, but in those who taught the truth; nor in those who related strange commandments, but in those who rehearsed the commandments given by the Lord to faith, and proceeding from truth itself. If, then, any one who had attended on the elders came, I asked minutely after their sayings — what Andrew or Peter said, or what was said by Philip, or by Thomas, or by James, or by John, or by Matthew, or by any other of the Lord's disciples: which things Aristion and the presbyter John, the disciples of the Lord, say. For I imagined that what was to be got from books was not so profitable to me as what came from the living and abiding voice. (Introduction to The exposition of the oracles of the Lord, as quoted in Historia Eclesiastica 3.39.3-4)
Papias' credibility is enhanced because of his method, not in spite of it. He diligently sought the accounts of first-generation Christian teachers in order to know as much as possible of what Jesus taught, which would necessarily include accounts that were not preserved in the 4 Gospels.
As already noted by Perry Webb, John's epilogue gives an explicit disclaimer that he has not recorded much of what Jesus taught (see John 21:25).
Neither Papias nor 2 Baruch are the first to record parables about unbelievably successful crop yields:
Yea, ten acres of vineyard shall yield one bath, and the seed of an homer shall yield an ephah. (Isaiah 5:10)
So the general idea long predates the first century.
As far as Gospel authorship is concerned, Papias is a valuable source, particularly with respect to Mark, but:
- Matthew - It is quite possible to argue that 2nd generation Christians believed Matthew wrote Matthew without appealing to Papias (see this presented as a deductive argument in my work here: Who Wrote Matthew?)
- Mark - the most critical information Papias provides about Mark is also provided independently by Clement of Alexandria (see Historia Eclesiastica 2.14.6, 2.15.1, 6.14.6-7)
- Luke & John - if Papias wrote about the origin of these Gospels, his writings on the subject have not survived
Re Eusebius - Eusebius had an ideological axe to grind - he thought Papias unintelligent because Papias believed the millennium described in Revelation 20 was literal (a position held by many Christians past & present), and this motivated Eusebius' cheap insults against Papias (Historia Eclesiastica 3.39.13). However, Eusebius' actions speak louder than his words: he quoted Papias repeatedly as a meaningful historical source (also, Irenaeus of Lyons, who knew the work & life of Papias far better than Eusebius did, held Papias' work in high esteem).