I think this is a bit of an oversimplification of Vance in Destiny 2, as well as what we knew of him in Destiny 1. As of Destiny 1, his idle dialogue implied that he was previously a Guardian, as opposed to being one during the game’s canon:
“I used to be a Warlock, now I follow a different path.”
“I began as a Guardian, I left to follow a different path. Perhaps you will too one day.”
In Destiny 2, Vance does offer an explanation for the change:
To be a Guardian! How I longed for it. After a near-death experience in which I lost my eyesight, I even believed that I had become a Guardian, that the great Osiris had granted me my dearest wish.
How young and foolish I was!
This need to “feel like [he] was enough” after a traumatic incident led him to be rather deluded when it came to his Light, though obviously he was trustworthy enough to gain a spot at the Reef and oversee the Trials of Osiris. The only person that says he was a Warlock is Vance himself- and, given the nature of unreliable narrators in Destiny’s lore, I don’t think this change is equivalent to an objective, well-sourced fact being altered. We as a community took Vance at face value, despite the Cult being, well, a Cult. That’s not to say that everything the Cult said or did in D1 is without merit, but it was made clear that the Cult had ulterior motives, and taking all information presented by them as fact isn’t recommended.
Regarding his character in D2, in Curse of Osiris, Vance is introduced during a stressful situation. As he says himself:
Brother Vance: The Vex are at my door. Osiris has not returned. Faith is all I have left.
The Cult’s- and by extension, Vance’s- mysticism is played off more jokingly than it was in D1 (“We followed the signs here, too. And by signs, I mean facts.”). But the Cult is still capable; it’s what gave the Guardian the ability to recover Sagira after she was damaged. They also managed to correctly interpret the Prophecies of Osiris, barring any new revelations.
As for Vance being fanatical, he still manages to be rather composed within D2’s lore:
“You may not know the legend of Osiris or ever dared our trials, but should we call on you, will you honor the heralds who earned this weapon’s honor and continue the hunt?”
His dialogue in-mission isn’t all of his character; in addition, there can easily be a difference in his actions as a blind civilian in a high-pressure situation as opposed to in a safe corner of the Reef.
Some of his lines can be viewed as a bit campy, but he’s far from a joke character in D2. There’s also the fact that he did have joke lines in D1:
“Why do you make me deal with this Shaxx, master? Is it a test?”
The “‘Folklore’ statement,” while hotly debated on sites like Reddit, was not made as an excuse to gut D1 lore. It’s an explanation for stories such as the Books of Sorrow or Palamon- they cannot be taken completely as fact, because their narrators are unreliable and prone to forgetting, if not outright omitting, information. Jaren Ward says so himself:
I’m writing this from memory - some mine, but not all. The facts won’t sync with the reality, but they’ll be close, and there’s no one to say otherwise, so for all intents and purposes, this will be the history of a settlement we called Palamon and the horrors that followed an all too brief peace.
And, as Christopher Barrett said:
All that was meant - the entries in the Grimoire are written within the context of the game world by characters who might have individual opinions, perspective, or varying sources. They are bits of lore and legend.
I don’t think this was a change made out of malice; it could be, much like the Rezyl Azzir/Yor debate, something that was always intended but instead taken as a retcon. Vance’s Light was only mentioned in his idle dialogue, something that shouldn’t be valued over D2’s lore.
I’d recommend this post of idle dialogue transcripts, although it’s incomplete.