I can say:
- I have a vague memory of something.
- I have a vivid memory of something.
So are they opposites?
I can say:
- I have a vague memory of something.
- I have a vivid memory of something.
So are they opposites?
In that one use (in relation to memories), it could be argued that they are.
I have a vague memory of something.
Means I can barely remember it but I can remember it a little bit.
I have a vivid memory of something.
Means I have a very strong, clear memory with lots of details.
In general, though, they are not opposites and would not be used together..
A photograph with "vivid colors" could not be compared to a photo with "vague colors"... I'm not even sure what that means. The common opposite would be "muted colors".
If you have a "vague idea" about how you're going to do something, the person who knows exactly how they are going to do it would have a "clear idea".
Vivid means stimulating, bright, loud, lively.
Vague means "imprecise" and is usually used for describing someone's words or communication. It isn't used to describe the visual look of something very much. I would not call a work of art "vague", for example, unless the work of art was intended to communicate something (like a sign) and failed.
Dull is a better antonym of vivid, as it can mean things that are not stimulating, not bright, not loud, not lively, "boring", etc.
In relation to a memory (of something), you could say that they are opposites with a measure of certainty, but there are instances in literature when the writer uses them as adjectives that do not exclude each other:
But imagination is "merely the vivid but vague association of images with passion," while reason, the governing faculty of physical science, associates images "according to facts observed in nature" (Jonathan Smith, 1994)
Sapir and Whorf's writings on this matter tend to be vivid but vague, as the following quotation from Whorf illustrates: We dissect nature along lines laid down by our native languages. (Paul Elbourne, 2011)
And especially this quote:
My memory of the events that followed is both vivid and vague. Vivid is the emotional climate - a sense of relief about the change, about being able to leave this accursed place, where we had spent almost two years - two painful, frightened, and hungry years. Vague are details of geography, of people. (Lilka Trzcinska-Croydon, 2004)
Still, the fact that this latter writer feels the need to explain why her memory is both vivid and vague indicates that "a vivid memory" of something would in many (or most?) contexts be in opposition to "a vague memory" of something.
Here's another example with "memory":
There is support of this kind for the idea that memory for trauma can be both exceptionally vivid and vague or disorganized, depending on how the study is conducted and what type of memory is targeted. (Friedman et al., 2014)
One more radical antonym to vague with respect to a memory (of something) is probably distinct:
Pete Doherty: 'I have a distinct memory of friends I had at school whose parents were, for want of a better word, bohemian.
It would be harder to say "my memory of that experience is both distinct and vague" compared with vivid and vague, IMHO.
With "vivid and vague" we can have a division of labor: a memory is "emotionally vivid" but "factually vague". But it's hard to have an "emotionally distinct" or "emotionally vague" memory, so this division of labor is less possible.
You are right. Vague is the opposite of vivid. Alternatively, you can use clear instead of vivid.