I'm asking this out of a mix of a personal and professional curiosity.

I suppose I should start; my personal opinion is that a prologue should be vague.

There should be SOME setup in the prologue for the "first half", but it shouldn't be at the forefront of the prologue. Keep it vague, but still hold the reader's attention.

No. Vague-ness is always a negative.
It should be clear and intriguing.
It should not be vague in the sense that you will make a phantasmagorical congloromation of shadowy images. You can choose not to show everything, or anything for that matter, but what you're not showing should be as clear as the things you choose to show.

A prologue should give a hook, something for the reader to look forward to, or to set the atmosphere for stuff to come. The vagueness, that you see is most likely caused from a sense of disorientation as you are presented with a small portion of the story with few points of reference. For example, an adventure story might start off with a sort of dark scene with the villain. It lets the readers know a bit of what to expect before the story jumps into its first chapter (most likely involving a character being pulled from their everyday life).

Vagueness is not necessary, it's simply a product of getting a point across to the reader, such as atmosphere or conflict without answering all the questions that the reader might have. In fact being clear (for example the intro to the 'Magic Duel' episode) can accomplish this very well. In that episode's prologue we learn that Trixie has some new amulet and she's out to use it for no good. Nothing particularly vague about that, it provides the hook to make the watcher concerned and intrigued.

Vagueness is not good or bad in and of itself. You might as well be asking if your prologue should be happy or sad. The answer is that it depends on what you want to achieve. There is no one-size-fits-all style of prologue.

Above all else, a prologue should capture the reader's attention and give them a reason to keep reading. Vagueness is a good tool because it offers the reader a grasp of the story that's strong enough they understand the world and the conflict, but weak enough so that there's more to learn as the story progresses. A prologue should be a hint and a taste of what's to come.

That being said, prologues are easy to screw up. Most of the bad prologues I've read are bad because the author misuses vagueness (where something isn't fully known to the reader) with blandness (where something isn't interesting to the reader).
The flip side of that is being overly-precise, where the prologue is turned into a mess of jargon and confusion. One version of this is where everything is explained right out the gate, turning the very first chapter into a massive info-dump of a research paper. The other version of this is where the jargon is never explained, leaving the reader lost as to what's going on and too frustrated to continue reading.
Don't do any of the above.

954580
I don't think it should be vague for for the sake of being vague, I lean more towards hinting at the conflict that will be established without giving it all away.
And if you don't know enough about the story to say what the primary conflict is, you need to work on your story more

Vagueness can be good, but like others said it should have a hook. One of the things I wanted to do for my prologue was give an interesting hook.

Its all subjective, but I personally don't need EXPLOSION ACTION MICHAEL BAY*** right off the bat to get my attention. I prefer something interesting in a prologue that doesn't turn it up to 11 right away.

Robert Jordan's The Wheel of Time has the best prologue I've ever read. I'd say check that out.

I say yes, but to a degree. You dont need to give a story's synopsis, but just enough information to set up the plot or do your job.

Im writing a story that is a tregedy, and in the prologue (about 200 words) i describe a mare,(no discription) walking jnto a classroom full of kids and is going to read them a storybook about a historical figure. It shifts to her thoughts on why she hates the book, because it ruins the heros image, but she still does it to remember the story.
Then the story starts up, 30 years previously, and the whole story is a flashback.

954580
Personally, I'm of the opinion that a prologue should usually never exist.

They should be a interesting hook, showing something from a viewpoint that the normal story doesn't, setting up some of the plot or conflict that would be difficult to otherwise. If used as that, okay.

All too often, what I find instead is essentially just 'Chapter One' by a misleading name, or a shameless, dull exposition dump.

As for if they should be vague...
954616 more or less says it.
If you're hinting at some mysterious plot which will be the focus of the story, of course you don't want to give too much away. If the evil mastermind of the plot is Fancypants, and finding that out is important, by all means, just refer to him as the 'refined gentlepony' or something, when he's talking to his circle of equally mysterious conspirators.

Conversely, if your prologue is a slice of action from a changeling invasion of Manehatten, or Discord breaking free once more (pre-Keep Calm) what do you get by being vague?

954921

You're like a lot of professional editors in that regard, many will simply ignore a submission if it has a prologue due to how poorly it's usually used.

IMO, there are only two reasons to use a prologue as opposed to simply having a "Chapter 1:

1. To outline a backstory quickly. This is critical in certain fantasy and sci-fi works that take place in their own unique worlds that so different from the stereotypical norm that it is difficult to understand what is happening without prior explanation. In these works there's simply no way to avoid having an info-dump, so it's better to get that out of the way at the beginning.
2. When the pace of the story is such that it becomes necessary to build expectation at the beginning. Such a prologue should foreshadow ending and present the core concept behind the story, and the ending should answer why it exists.

954921>>955327>>954587>>954918>>954843>>954822>>954618>>954618>>954599>>954587>>954591 thank you for the input guys

956012
also worth noting, my prologue is a part of my first chapter.

I think it depends on the point of the prologue. Some prologues can simply be used as a sort of author's notes prior to reading. Other times, the prologue serves a unique, separate purpose from the story itself. For example, I've seen prologues used to communicate a conversation between the author and the main character of the story at times.

I used my prologue as a preface. I wanted to establish a time line. That the events that happen afterward happened just after a major event in Twilight's life, but it was too short for me to call it an actual chapter. It was a small bit of backstory in the not-so-distant-past, to help readers place the current events in the here and now.
Can my story exist without this prologue? yes
Is it helpful? I think it helps and in the great scheme of the whole story it doesn't really hurt either.

Look at the prologue for Deus Ex. Copy its essence. Bam, perfection.

To me, a prologue should set up everything else from that moment, but only let you see a small slice. Give the reader's imagination something to work on - and worry at. Something to get their interest, something to make them want to see more, something to grab and hold them.
Not very easy to do, as I found out, but when it happens, it's wonderful.