Well, the vanilla you see on yogurt and ice cream cups refers to the flavor.
The definition you are asking about talks about something else.
It comes from the basic meaning of "vanilla", namely an ordinary flavor of ice cream or other dairy/bakery products, but has evolved to mean the default option that comes with no extra features.
So for example if someone says It's so hard to pick a laptop.
I think I am going to go with the vanilla version.
They are basically saying: "I will buy the one with no special features or outstanding characteristics.
" This usage has nothing with taste.
It comes from the notion that vanilla is the most common flavor among all the flavors.
ell.stackexchange.com/users/61125/Eddie Kal
Vanilla can be a synonym for plain , but that's not what's happening here.
With yogourt, vanilla is referring to the noun , as described by Merriam-Webster: 1
b : a commercially important extract of the vanilla bean that is used especially as a flavoring
ell.stackexchange.com/users/73781/Jason Bassford
The confusion seems to arise from the fact that in your quote from Merriam Webster, you provided only the second of two adjectival definitions.
The whole definition is: vanilla adjective Definition of vanilla 1 : flavored with vanilla 2 : lacking distinction :
plain, ordinary, conventional In the case of Yogurt, the first is in use.
And in that context "plain" simply means "unflavored" (although anyone who has tasted plain yogurt will tell you, after they stop grimacing and smacking their lips, that it certainly does have a flavor!)
Outside of Yogurt, or food flavoring in general (and maybe the sub-field of botany dealing with Mexican orchids)
"vanilla" is just a synonym for plain; i.e. lacking distinction, ordinary, or, to throw in another example of this kind of thing, "common or garden".
The reason "vanilla" took on that second meaning of plain/ordinary etc, is that in foods such as ice cream, yogurt, and custard, vanilla was by far
the most commonly used flavoring, so much so that it came to be regarded as...well, plain, ordinary, and so on.
Other examples of this kind of thing are the aforementioned "common or garden", and "box, standard" (sometimes mispronounced as "bog standard")
ell.stackexchange.com/users/8719/tkp
In my experience, vanilla in that sense is not used for food stuff.
As you correctly notice, that would lead to confusion.
In other areas, where no confusion with the actual taste of vanilla is likely, it often means unembellished , without any added stuff.
So a vanilla operating system on your phone means that there are (almost) no pre-installed apps from your provider or other parties.
A vanilla car would be a basic version without any extra options.
ell.stackexchange.com/users/2992/oerkelens
To answer the question in the title, vanilla yogurt is sweetened and tastes like vanilla, while plain yogurt is unsweetened and doesn't have added taste.
ell.stackexchange.com/users/51492/Stephen S
Merriam Webster  explains  vanilla  (when used as an adjective) as:   lacking distinction : plain, ordinary, conventional   It's not obvious why  vanilla  has such a meaning, and why  plain  is listed as its synonym. As you can find in the grocery store, vanilla yogurt and plain yogurt are two different products:     So I'm wondering what's the exact difference between these two words and how to use  vanilla  correctly so that the salesperson doesn't hand you the wrong yogurt.
What's the difference between "vanilla" and "plain" when talking about yogurts?
ell.stackexchange.com
<meaning-in-context><word-meaning><word-difference>
ell.stackexchange.com/questions/176388
ell.stackexchange.com/users/44598/Cyker
This comes from the basic meaning of "vanilla", namely an ordinary flavor of ice cream or other dairy/bakery products, but has evolved to mean the default option that comes with no extra features. This usage has nothing to do with taste, but stems from the notion that vanilla is the most common flavor among all the flavors. Vanilla, apart from the flavor, is used for something ordinary.
Vanilla used on food refers to the actual flavour. Since vanilla is generally seen as the most basic and common flavour, the usage has been extended to mean the basic model of something.