You can relax ions using ISIF = 2 for different sets of volume. 

Dear Reza,

I don't know the version of VASP you use. For current version I suggest you set ISIF=3, which allow you to relax both volume of cell and internal ions. However, the shape will also change. Therefore if your angle is not the lowest energy type, I am afraid that VASP will change it although not so much.

If you must keep that angle, ISIF=7 maybe the best choice.

The latest VASP manual describing ISIF setting is show in:

http://cms.mpi.univie.ac.at/vasp/vasp/ISIF_tag.html

Hope this website will help.

Of course you can write a script yourself, if you really really want that angle...

Cheers, Yixian