Breaking news !!! I find the solution.
The problem is that the var_1 is declared as factor so it's not possible to add factor and numeric value. I added as.numeric to var_2.
Now it's possible to use ggplot to have the position dodge with stack position. Cool !! 
...mapping = aes(x=as.numreric(var_1) + barwidth + 0.01 , ...
