Interesting discussion. I'll butt in if I may.
Simple protic acid/base neutralization concepts won't work for this system of ferric chloride in water, in the presence of air (oxygen).
The system is actually very complicated. But think of oxidation (as in burning), which is what etching and rusting is.
As I understand it, an approximate summary is that, ferric chloride (also known as iron(III) chloride) can covert to iron oxides and iron hydroxides (rust) in the presence of oxygen and water. This releases hydrochloric acid, which attacks the iron in the blade, making hydrogen, and ferric chloride (and iron(II) chloride also known as ferrous chloride)
again.
Repeat the conversion of the two types of iron chloride to hydrochloric acid using oxygen from air dissolved in the solution and continue. .
The actual concentration of acid may never be large enough in the bulk solution to be measurable, or considered acidic. But it never gets used up, as the cycle continues to replenish it. But oxygen and the blade continue to be consumed to make iron oxides and iron hydroxides. As iron hydroxides build up, the
bulk solution will become somewhat basic. But corrosion will still continue until it gets very basic.
Masochists may get some idea of what's involved by looking at the summary section of this article on corrosion which describes the role of iron chlorides:
http://nautarch.tamu.edu/class/anth605/File9.htm
What is needed is to remove all traces of ferric chloride and ferrous chloride because they let the chloride ions be used over and over again to attack the blade as hydrochloric acid. So the chloride ions are what make this system so effective at eating iron.
Excess ammonia solution (a base, but also a good ligand for iron ions) probably converts nearly all of the iron chlorides to less corosive ammonia complexes of some sort. And any free hydrochloric acid will be neutralized as ammonium chloride, a salt.
Excess vinegar (solution of acetic acid, but the acetate ion half is a good ligand for iron ions) probably converts nearly all the iron chlorides to less corrosive acetate complexes.
Simple mechanical rinsing effects at work too, of course.
Either vinegar or ammonia, likely effectively no more ferric chlorides. But any chloride salts of whatever type left have potential to start rust and generate some iron chlorides, so rinse well with water. Undoubtably things are more complicated than I describe.
My suggestion:
Use more than one treatment with fresh vinegar or ammonia (some tiny proportion of the iron chlorides won't convert--but a tiny proportion of a tiny proportion gets pretty close to zero). And final rinses with water.
Scrub surface if any thing could be trapped under any crud on the surface.
I don't think that baking soda (sodium bicarbonate) solution will convert iron chlorides to alternate, benign iron compounds as vinegar or ammonia will. Sodium bicarbonate makes a buffer solution, which means if enough is disolved, it self-regulates its pH until too much is used up reacting with acids. It holds pH 8.4, which is according to the alkaline inhibitive solutions section of the linked article not basic enough to inhibit corossion if chloride ions are present. Since it's a buffer, a solution cant be made any more basic than 8.4, no matter how much is dissolved. Household ammonia is a stronger base.
Lastly, I would not use vinegar and table salt, which one may come across in some discussions of "de-oxidants" for copper etching. That actually makes some hydrochloric acid. Which is why the mixture is so great for cleaning copper. Remember that those etching fragile copper PC boards want a totally clean, oxide free surface for later fabrication and they may do some things especially for that reason. Any very thin oxides that form on a blade by stopping an etch will be removed by polishing or soldering preparation.
