AMDFusion...don't ask in download section.
Here is your answer though.
EG:
Footholds:
1 2 3 4 5 6 7 8 9 10 (5 is the middle point where you wanna vac to)
You put the previous value of 1,2,3,4,5 to 0 and next value of 5,6,7,8,9,10 to 0.
(Pvac)
If you wan Fvac, add in the force for all footholds except 5. (So that you won't slide to nowhere)