So... this is actually a really important piece of a conversation that needs to happen (talking about the role and implementation of stealth in League) but you are going about it the wrong way - you'll never get a Rioter response with your tone, or even the significant upvotes necessary to get their attention.
But... to address your grievance: what those characters (Vayne, Kha'Zix, Akali) are doing is not, in essence, stealth. It is only invisibility. The terms are not synonymous or interchangeable.
The problem the developers (the Rioters) have is that their definition of stealth is irrevocably flawed and as a result their methodology regarding stealth gameplay has been haphazard and piecemeal. Until they make changes that are in accord with the actual definition of stealth, they will continue to run into problems with certain champions and League as a whole.
Stealth has one definition, and that definition is consistent even when stealth appears in League, despite Riot's erroneous use of the word:
Cautious and surreptitious action or movement; the quality or characteristic of being furtive or covert.
Stealth and invisibility are not synonymous or interchangeable. I can't stress this enough. You can be invisible without being stealthy, although invisibility is usually a prerequisite for stealth. Camouflage, in League, is invisibility with proximity as a parameter. Camouflage, IRL, is not a type of 'stealth' but something that can enable stealth.
Stealth is not a mechanic (a mechanic being the player-input via button presses.) League's code renders our champions invisible to the enemy, but stealth is something that we, as the player, must actively and consciously engage in, via our avatar (mine is Shaco). It is a mindset or playstyle i.e. behavior.
The word 'stealth' is derived from the word 'steal', which you can check in the dictionary, has this as one of the definitions:
To move, convey, or introduce secretly; to accomplish in a concealed or unobserved manner.
Almost every game that we call a 'stealth game' operates with this definition of stealth in mind. In fact, the term 'stealthed' does not exist in any dictionary, as it is a recent development brought on by a few very specific games, notably World of Warcraft. In WoW the Rogue ability 'Stealth' was just invisibility because the game's systems did not really support actual stealth gameplay, but the term 'stealthed' was shorthand for invisibility (fewer syllables). And tens of millions of people played WoW, and the term spread, and some of those players were future Rioters, and like an infection, it spread further, as now League's players use the word.
And we can witness the effect this erroneous use of language has had on the community:
Akali Is So Hot said: Eve could be stealthed forever.
DuskDaUmbreon said: Rengar: Stealths with ultimate.
Jungle Lux God said: Those champions also had stealth for an extremely long duration.
Steel Blossom said: The thing for re-stealthing assassins...
Dead Flag Blues said: I still think that stealth on ranged champions is extremely unfair.
Outside of the gaming community, no one has ever spoken of stealth in this fashion. You can use stealth, be stealthy, or behave stealthily. You can modify some words, e.g. stealth-bomber, which signifies the word as a characteristic of that thing but 'stealthed' and 'stealthing' are not real words.
I don't mean to rant, but I think this is important. Riot needs to create an ecosystem of items and mechanics and elements suitable for stealth gameplay, otherwise the stealth-champions will prove problematic in some way. I could go on, but I'm tired.