maxscript transparent background (alpha)

I need help to make transparent background via maxscript. I know that its just one preperty from the Preferences->Rendering->Background->Use Environment Alpha.

I'm using vray 2.40 for render - so its V-Ray frame buffer

Here is my code:

f = "D:\\1.max"
    mergemaxfile f #select;
    renderFilename = getFilenameFile f + ".png";
    pngio.setAlpha true;
    pngio.setType #true48;
    rendOutputFilename = "D:\\"+renderFilename;
    rendsavefile = true;
    bm = render();
    bm.FileName = "D:\\"+renderFilename;
    save bm;
    delete selection;

So after this code i have a picture, but background is BLACK (from background color - environment)

But if i would save this render mannualy by my hands from frame buffer - the background would be transparent... how to solve this problem? i need transparennsy from script!(