ok - here´s a follow up to the last problem. as discussed before i got the alpha to be transparent behind the window with the "affect alpha" checkbox in the glass materials reflection layer.
unfortunatley i ran into an new problem with that kind of setting. to understand you maybe have to know i´m using the vray sun and hdri method trick with the unchecked reflection slot in the environment rollout. now when i try to put my backdrop behind the model in photoshop the glass seems to lose its reflections in the areas where the background shines through the window. looks not very nice and destroys the illusion of photorealism. in fernandos tutorial on youtube the reflections seem to stay. i tried so many things in the meanwhile that i think even if the solution was obvious i wouldn´t get it anymore.
another minor thing that happens when you save to png are the sharp white borders around the model you get in the alpha channel of the png. funny thing, if you crop the model to the alpha pass (not the alpha mask of the diffuse png)you can get rid of them easily.
if it´s uncelar what i´m talking about let me know and i´ll try to illustrate my problem with some pics tomorrow.
again thx for any pointing into the right direction!