Wait, each batch in the OP was aired out and allowed to oxidize and everyone was bad tasting, but nobody noted that there is a difference between steeping and leaving a bottle open?
Good advice, but I think the whole oxidation thing should be brought up for other newbies' sake.
I think it is a little unfortunate choice of terminology to say "steeping", because steeping is defined as a process that involves liquid extraction of flavors from a solid material. Now if someone is making their own tobacco extracts, or more commonly probably coffee extract, they may in fact be steeping. If you are mixing liquids in a bottle there is no steeping process because there is no solids and no extraction occurring.
But that is being pedantic, steeping is the word that most people seem to have adopted.
There are a number of things going on, and it is IMO too complex to really make any sort of concrete statements about what is really happening. But we know that chili and spaghetti sauce is never really that good the day it is cooked. It needs to sit in the fridge overnight and be reheated, and by far the best is the stuff that has been in the freezer for a while as long as it has not become freezer burned. In culinary circles the term "marriage" is used, which is presumably a process of blending (flavors soaking in and out of sauce, pulps, meat, beans, etc as well as some chemical reactions occurring). Similarly, many wines and most spirits require lengthy aging, properly served red wines are usually allowed to "breath" after opening while white wines are not etc etc etc.
I think when people quick age capped, two basic things are happening - blending because some components do not mix all that easily, and some acceleration of reaction of the mix, a sort of marriage if you will. I often allow my mixes to heat capped and this is called "reflux" - you can see vapors rise and condense and then drip back down.
Uncapping initially is I think most people attempting to lose some objectionable perfume, chemical, flowery, etc characteristics some of these flavors have. I also do this, but do not store anything long term uncapped afterwards.
Many complex things are happening in subsequent aging, who knows what?
As far as oxidation goes, it certainly happens. Many flavors, example vanilla family, are methoxy and hydroxy benzaldehydes which are rapidly oxidized. In fact these types of compounds are the basis for anti-oxidant additives - they "scrub" oxygen out of a system by preferentially reacting with oxygen before other components can.
Whether this is a good thing or a bad thing just depends on your taste. You may prefer the oxidation, you might also want to experiment further and you may find that you prefer to allow some basic tobacco mix to age, then add for example, some ethyl vanillin to it just before you vape it.
However, the flavors are present in small amounts, I have never seen anyone using any kind of vial that is entirely moisture tight, and not even remotely close to airtight, much less flushing the vial with a blanket of inert gas like nitrogen or helium, and the glycols themselves will dissolve a generous amount of oxygen. I doubt capped versus uncapped has much if any impact on the oxidation of at least some of the commonly used flavors like the vanilla family - it is gonna happen, period.
Long term uncapped aging certainly loses some of the heavier aromatics over time. But another effect, I don't know if it is already well known because I have not searched the forums extensively, is an uncapped bottle is absorbing a considerable amount of water over time, which is acting as a diluent as well as changing the vaping properties, chemical reactions of the mix etc. I have not studied it closely and cannot speak about undisturbed capped bottles, but a typical small dropper bottle in active use will certainly increase in water content.
I personally forego the cost factor and buy flavorings and especialy PG-VG base in small quantities.
None of these considerations are inherently "good" or "bad" - taste is very subjective. Nor does the fact that what is really going on is far too complex for us to get a real handle on it mean that this is all an exercise in futility. It simply underscores the imperative that whatever you do, learn to do it consistently and keep detailed notes Just because you can never know what is really "in the mix" does not mean you cannot make it again with a good degree of success (until of course your vendor screws you, at which point sadly your holy grail of vapes may be lost forever, but what can you do?) ;-)