hello
I want to apply a delayed force into the system, i.e that the force which renders back to the device corresponds to a delayed position. how do you suggest would be the best way to do it?

I thought to build a 'buffer' vector according to the length of the desired delay: for example, for 100 ms delay, to create a vector with 100 elements and updated it every 1 msec by using the functions- erase(begin()) and push_back(z)
but I am not sure in where to define the buffer vector, and which 'cVector3d' to choose for those manipulations.

CHAI3D does not support time delays as they are generally undesired effect! :-)

This said, if you are working on delay problems and would like to model the effect, you could, as you suggest, use a buffer where you store the position and/or force data with a time stamp. The haptic device positions can then be feeded to the simulation and the computed force applied to the haptic device with the time delays expected.