What plumbing code is in your area?
One of the model codes allows as many AAVs as you want, the other restricts you to a single AAV with approval from the AHJ.

California Plumbing Code. :) They do their own thing, and struck the exception for air admittance valves in the latest version. But that's why I'd feel okay with it for a fix if there are no other big reasons not to do use one, since we wouldn't be pulling a permit for something small like that anyway---just trying to make the situation better than it is now.

"...but we'd like to correct the problem."

"There doesn't appear to be any steam/water damage where the vent terminates now."

If you don't have a problem, what IS the problem? (apart from code issues)

Venting in an attic or any enclosed bulding cavity is not a good thing as typically molds are the beginning of a spiralling descent to the dark side.

The AAV will allow the drain to function without siphon, but since it is a mechanical device it can fail over time.

Better to go through the roof.

The bigger problem with a vent ending in an attic is that sewer gas is smelly and can be flammable/explosive.

The sewer gas was the concern, and what I was hoping the air admittance valve might help address. If it's not a good solution, though, we'll just wait until we can run it through the roof. I don't anticipate any problems with mold and the like suddenly developing if we haven't had any in over six decades, so that part doesn't really worry me.

An A.A.V. (air admittance valve) would be your solution. It would definitely prevent the build up of any sewer gas in your attic. It would also prevent any unwanted rodents, insects from entering your plumbing system. Simply make sure the vent pipe is approximately 6" above any insulation material, glue a female adapter and screw the A.A.V onto it. Check with www.rectorseal.com. They manufacturer the Magic Vent A.A.V.

Actually the solution is to just leave it alone.

As it is now it met the code at the time of installation and therefore it still remains code approved. If you alter it in any way that constitutes "new work" and you would be required to insure the entire length of that vent is up to the current code.

It's not likely this is a vent only for a bathtub. Don't cap it with an AAV. Why not? For the reasons lazypup mentioned, and for more reasons. One is that an AAV only transfers air one way not both ways, and this hindrance is why it's not an adequate solution in many Drain systems even when it's working as designed.

Thanks all--not sure how this old post got reignited! At any rate, we decided not to do anything about this until/unless we renovate the bathroom in the future, at which point we'll be moving plumbing anyway. This is indeed a vent just for our bathtub---we have a small house and the toilet and kitchen/bathroom sinks (the only other plumbing in the house) are separately vented, for whatever reason. We're also not too concerned about code since it's an old house and things are all over the board; the concern was really just making sure that we weren't damaging the house or ourselves in any way by having the tub vent into the attic.

Is the attic ventilated to the outside?

Yes---it has a window (no glass, just the type with open wood slats) and a number of roof vent caps. Plus plenty of air leaks that should probably be sealed at some point, but aren't going to be any time soon. :)