This man leaves everything vague
Character strengths? Vague
History? Vague
Whole world building? Vague
If by "Vague" what you mean if not knowing every single information about a character/event/place/etc, then that is not possible. By that definition essentially all literature is vague, since every single detail cannot be shared.
What Oda does is leave the outcomes vague, until he's ready to reveal them later down the line.
Sabo incident at the reverie - Left vague as to what could've happened, and then later down the line he sheds light on it.
In story time it's only been few weeks at best since the reverie, so not much time has passed. This Blackbeard vs Law / Shanks vs Kid aren't vague, he just switched from them and will just show us the results.
I think what he does in terms of stating an X event happened, and then reveal the results of it later down the line is fine, as those are side events. Even though some of them are more interesting. But the story he wants to write on is Luffy as the main piece, so all of those events will be left that way. He's not writing a story like KINGDOM where Shin is the MC, but you have characters who get their own arcs without Shin even being involved.
It's what comes with the territory of writing this type of a story. Now if one piece history/character strengths are left vague by end of the manga, then sure that's an issue. But we're far from the end.