I would think that vent might be over the septic tank itself. Also, if the septic tank and drain field are still in use (i.e. the house is not connected to city sewer system) you absolutely can't plant anything with a deep root system over or around the drain field. No trees or shrubs.
I have a large raised bed over my septic field, but it's an old field and at least a foot underground. The raised bed is 18in high, too. So I am confident that my shallow-rooted plants are not getting any roots down into the septic field.
You can find out the extent of the septic system by just taking a sharp rod or a piece of rebar and poking it around in the grass back there. If you encounter gravel under the soil about 8in. deep, that's what the septic field is bedded in.
You might also be able to get a lot survey from your county zoning office, and it will show you where the septic tank and field are.
