Obviously we can only guess.

If this is an older home where the large cast-iron pipe runs straight up to the roof, I would guess that this too shall pass.

If the ball is still complete, it'll probably be floating in there, since I'm no expert I don't know how the vent stacks go with levels of drains etc - I would assume if it went all the way down, it'll eventually hit the water level, and sit there.

Is it possible (albeit unpleasant) to look down the stack and see if you can see it? Chances are it could be wedged somewhere, if you see it, you may be able to figure out a way of snagging it, with a long pole with a screw on the end you can force into the ball, for example - or a long piece of plastic pipe hooked to a shop vac suitable for wet/dry (in case you vacuum up some effluent!) which you could lower down to the level of the ball.

I am not convinced it 'would pass' and think perhaps, the other suggestions notwithstanding, you might want to get a plumber to open up the inspection eye/trap where roto-rooter would do their thing assuming this is where the ball would end up, if it's floating at the highest level, if the eye is below that, the ball would hopefully make its way to the eye/hatch. I suppose a roto-rooter sort of treatment might with a bit of luck, force the ball back towards the sewer, where the larger diameter ought to ensure it's no further trouble.

The ball won't be floating because there is no standing water in drains. If it went all the way down it is gone, no further action needed.

I would'nt spend any money where there is not a definite problem.

How do you know the tennis ball went down the vent pipe? Did you actually see it go down the pipe, or are you just guessing that it went down because you can't find it?

i can honestly say tha talthough this sounds far fetched, i have seen something similar happen. the house we used to rent had 4 clean outs around it. teh "plumber" made them above ground, and one kept getting the cap knocked off it. i was throwing a tennis ball on the house one day and it came down and went perfect into the drain line! we had no problem for a while, then one day the lines started backing up. when he snaked it out, there was the tennis ball. it hung up on a tree root and slowly collected TP and other not so nice stuff until it clogged the 4" drain line completely.

even if i had called him to snake ti when the ball went down, it would have cost the same as doing it later when it had a problem. so save your money and wait to see if any drain back up. no problems, no call outs.

Just got a $1,200 bill. I knew it went down but thought it passed. Nope.

stack had a horizontal line.. ended plugging after 2 yrs. Fml

If you block a vent, get the blockage removed immediately. You don't want to stop venting poison sewer gas.

If the ball is still complete, it'll probably be floating in there, since I'm no expert I don't know how the vent stacks go with levels of drains etc - I would assume if it went all the way down, it'll eventually hit the water level, and sit there.

Is it possible to look down the stack and see if you can see it (albeit unpleasant)? If you see it, you might be able to snag it with a long pole with a screw on the end that you can force into the ball - or a long piece of plastic pipe hooked to a shop vac suitable for wet/dry (in case you vacuum up some effluent!) that you could lower down to the level of the ball.