I strongly believe after a long time use that the vertex k8400 need some major upgrades on the extruder/bowden side.
First, it is mandatory to shorten the bowden and reposition the extruder as shown by Hardbats
Second, great improvement on filament loading thanks to the user
In my opinion, a step further is to add a piece of bowden inside the extruder and near the gears to minimize the free movement of filament, in particular the flex ones.
Therefore, I edited in blender the Blackwolfin upgrade, I hope you don’t mind, Blackwolfn.
I positioned a pipe in which the bowden would fit, making a compatible slot in the second half as you see in the pictures. The length of the piece of bowden needs to be trimmed accordingly and by eye, as shown in the pictures.
Those two halves are named Base_1 Top_1
As a second modification , I made a second set of things with an enlarged nut hole in case somebody want to change the bowden clip with another one, with bigger nut (9,8 mm instead of 7,8 as the original one, see pictures). If necessary, the hole for the bowden clip on the top of the extruder has to be drilled a little bit, as the other holes in the two halves, no big thing.
Those two halves, with bigger nut, are named Base_2 Top_2
As you see, the Tops need to be flipped fro printing.
As a suggestion, print an additional extruder in case of need.
I can give you the blender file if you want to make additional editing