One way to move from a "vague idea" to a doable paper topic is to write your vague idea at the top of a page, and then, briefly:
Do any of these aspects seem especially useful with regard to your topic? Do any common themes appear? Keep going back to it as you read up on your topic.
Your paper's focus and thesis should strike you and your reader as interesting.
Exactly what is "interesting" depends on both topic and scholarly field, but it usually involves something the reader has some prior awareness about, but with some twist: a previously unexplored aspect or an unexpected finding. Another way to put it is that interesting is the middle ground between obvious ("duh") and meaningless ("huh??")
One way to think about an interesting approach to your topic is to think of your topic in terms of a contrast:
Something that appears _____ is actually ______.
...universal ...true only within a culture (sociology, anthropology)
...healthy ...unhealthy (medicine, psychology)
...new ...similar to the past (history, politics)
...just . ..unjust (ethics)
...a causal relationship .. .just a correlation
Any of the above patterns can work in reverse as well.
< Ex. Something that appears to be important may turn out to be unimportant: " Contrary to popular belief, there is actually no significant relationship between stretching before exercise and reduction of injuries during exercise. "
More examples of thesis statements and how to write them: