Not sure if this will help because you said a rod wouldn't work attached to the ceiling? But what about a curtain track? They attach to the ceiling, but are pretty narrow so they may fit in between your window and the vent.
Something like this:
I think they make curtain rods that have heavy duty adhesive on the ends instead of screws, so that you could put the curtain rods on the ceiling. The curtains would still hang vertically, just the hardware would be on the ceiling.
You could use some screw eyes (in either the ceiling or the wall), twist a medium weight wire taut between them, and hang curtains from that. Plus it's really cheap. If you have a very lightweight fabric for your curtains, it should work out fine.
If you want something a bit more polished - ikea has one or two versions of curtain hanging devices that is, basically, the same concept. If you end up needing to go more sheer/lightweight for curtains than you're comfortable with, don't forget that you can also get those privacy 'films' to put on the window also to cut down on being able to see in -- but will still let you get lots of light.
I like mumkimum's idea on the screw eyes.
I'm subbing because we have something at our house that would work for you. I'll try to take a photo and post it tonight. It is curtain rods, purcahsed at Ikea. Looking at your photo I think you have room between the wall and the vent for what we've done in our bedroom.
You could use something like this and hang it where the blinds are hung: