They do not have to have values, because thoses are the vectors that are calculated by vec_accelerate.

The first two parameters are used as return values, thus you would use the distance vector for your c_move instruction.
The speed vector is an input and return value at the same time, because it is used for the next calculation iteration of the vec_accelerate and also modified by it. At the beginning of an acceleration the speed of an object is zero (in most cases). You could set it to something nonzero if your object is either already moving or if you want it that way (for whatever reasons).

the last two parameters (accel and friction) are just used for input and calculation.

Hope this clears it up a little bit.
By the way: This is all covered in the manual:
http://www.conitec.net/beta/avec_accelerate.htm

But I also admit that sometimes the manual is a litte bit confusing (and for beginners maybe even more).

Hope you are not mad at me now

edit:
The calculation is explained more detailed at the accelerate instruction (which is the same but just for one dimension):
http://www.conitec.net/beta/aaccelerate.htm