Hi, if I want to move a vertex in 3d space 1 unit to the left side (in 2d the X axis) I would add a

Vector(-1,0,0)

if the vertex is on a plane Front orthographic. But if the point is on a plane somewhere in 3d viewport (rotated) and I want to bring the vertex 1 unit to the “left” side along this plane, how can I do this with Python and in general.

I know the normal of the plane and I know that the vertex that I want to move is on the plane.

Thx a lot for the code, this gives me a good starting-point and I understood it. Actually my wording was not correct, I talked about a plane, but I didnt mean a plane-object (mesh), I meant a virtual plane in the viewport (view plane).
But of course it is the same concept: What I need is the inverted world matrix of the viewport plane (perhaps I can ge this from the region3d?), multiply it with my normal vector (BTW: Isnt this the @ operator for multiply since Blender 2.8) and then I can add it locally, right?