Change the length of your parameter month to 2. Now it is three long and the result of your concatenate would look like 199700302 (example).
What are you trying with, what is in vbap ? where did you select this vbap ?:
if Is_tab-erdat+4(2) = vbap-erdat+4(2) and Is_tab-erdat+4 = vbap-erdat+4 .
And your loop as well you write
loop at t_vbap into w_vbap
And then you are checking ls_tab ?