If you're using pure tungsten filaments, they will not be poisoned by air at any time whatsoever, but they will *burn out* if they are heated to red hot or hotter in the presence of air. 1E-4 torr is kind of a minimum vacuum for this kind of thing, they won't last very long at that level but in this kind of experiment you'll probably physically break them first. (Tungsten that has ever been heated to glowing is extremely mechanically fragile.) You can tell that the filament burned out from air if there's yellowish-whitish oxide on the inside of your chamber near the filament. The higher the vacuum, the longer the filament will last -- go to 10E-6 torr if you can!I will only put current through the filament under vacuum (e.g. 1x10^-4 Torr). Will the filament be poisoned at atmospheric pressure in air even if there is no current through it?
Pure tungsten filaments work JUST FINE. They just need to be white hot. They are resistant to poisoning unlike other filaments.
If you are using cathodes from vacuum tubes that are coated... who knows? There are different coatings. I am not sure of the details of all of them. Some will hate air, others will not. Definitely don't heat them in air, that will destroy them. I would not bother messing around with this.
The third option is the yttria-coated iridium filaments from ion gauges. These are designed to not be poisoned by air, and can operate at fairly high pressures (even 1E-3 torr!) without rapidly burning out, although they will still have a longer lifetime the better the vacuum. However, they can be poisoned by various chemicals -- this is why tungsten-filament ion gauges are still used, since they are poisoning resistant.
A big part of what is going on regarding feedthroughs is that much work happens at high to ultrahigh vacuum, 1E-5 torr up to "how many zeros do you want". (and how many days do you bake your all-Conflat chamber while ion-pumping it!) I have worked with special motors that use high voltage -- they have a label saying that you can use them at atmospheric, and you can use them at high vacuum, but you cannot use them at between 100 torr and 10 microns because they will have glow discharges inside. Lots of stuff is assumed to just be for high vacuum. You may have seen Doug Coulter's posts on making special feedthroughs for fusion -- in his view, most feedthroughs are not designed to work with medium vacuum and glow discharges in general.
On the other hand, in a complex setup where there are not just simple anode and cathode plates or spikes, but a bunch of weird shapes... things get weird, and often there will indeed not be discharge in places you don't want it.