I made a new picture to explain what I mean. I want to move a vertex over an edge, but the edge should remain a straight line, because else my model’s messed up. So is there a way to lock an edge in some way, so that I can move the vertex over it, without ‘bending’ the edge. The picture shows what I mean.
If this doesn’t exist, maybe it’s not a bad idea to implement it into blender?
Just a thought…
Thanx in advance! 
Sure.
Select a vertex on either side of the vertex you wish to slide along the edge (doesn’t really matter which one)
Shift_s -> Cursor to Selection.
Press the dot [.] key on your main set of keys on your keyboard, or select the “crosshair” icon on the 3DWindow header to activate ‘Scale/rotate wrt cursor’
Reselect your vertex you wish to move and press [s].
Return ‘Scale/rotate wrt’ mode to the default by pressing the comma key [,] .
That’s greAt! Thank you very much. This will prove very useful to me. 