Is this a basement toilet that was installed after the house was built?
If so, then it's not a venting issue at all. It's because the main
drain line from the house is partially clogged. When was the last time
you had that drain line cleared with a plumbing snake?
If it's a basement toilet that was added after the house was built,
there's no requirement to install a vent stack for that toilet. All
that's required is that the toilet's drain pipe be connected to the main
drain line from the house to the sewer line or septic tank. In that
case, if that main drain line is partially clogged, then that drain line
can fill with water during a flush. As the water then drains away past
the partially clogged section, the suction created behind the draining
water can suck the water out of the toilet bowl.
If you can't remember when the last time the main drain line from the
house was cleared with a snake or jetter, then I'd do that before you do