Ok, so. Theory time.
Jack's visions are clearly not just the Irumodic Syndrome (or however that's spelled). So, what are they?
Our data points:
1) Jack has the Syndrome. This means his brain is different and he is potentially prone to hallucinations or visions.
2) The changelings want him in a bad way.
3) The changelings stole Picard's body (presumably from after his death in S1, when he relocated to his positronic current residence) from Daystrom.
4) Every time Jack has a vine vision is IMMEDIATELY after making direct, physical contact with a changeling.
Theory: they aren't visions at all. They are communication. Jack has, without his knowledge, picked up a rebel changeling. It is riding his body around like a carriage, using him to hide from it's fellows. It is possible it is even some kind of next gen changeling, like a child or evolution (fitting the season's legacy themes).
It can, because of Jack's Irumodic Syndrome, make a limited connection to Jack's brain. Allowing it to, for example, warn him of other changelings or direct his fight against them. However, by nature, when it makes contact with other changelings it is driven towards the Great Link and unification. Jack's visions are like an empathic echo of that sensation. An instinctive drive to be made whole, reflected in surreal imagery and only possible because of the weirdness of Jack's brain.
What if Jack picking up this rider is even why the changelings stole JP's corpse? Trying to understand the connection?