You're not supposed to have a vent discharging within (I think) 10 feet of a window like that. I've HEARD people running the vent pipe up the side of the house and chimneying above the roof line. There's a high tacky-factor to that, but it could work.
Why can't you go up the tub wall into the attic? There you might be able to tie into an existing vent penetration through the roof.