The slope behavior is "dynamic" by default. It means the slope will vary if you alter the sector heights, and in the case of vertex slopes, the "vertex" mobjs as well.
Our slope documentation is a little incomplete at the moment, bear with us; hopefully I fixed our info on dynamic slopes a bit just now.
EDIT: Mind you vertex slopes are NOT dynamic by default so I'm aware, you need to toggle on the No Tails flag on their linedefs to turn it on. And then you also need linedef type 799 for the vertices you want to move to set the z positions of them or something. I don't use them enough to know I'm right or not though mind. x_x
The Slope article on the wiki is now updated with information on dynamic slopes.