vital evidence
vital information
vital clue

And they revealed a taxi driver may hold vital evidence about events that night.

He could even use that as his excuse for withholding vital evidence.

In the absence of witnesses, the 911 call was vital evidence for the prosecution.

Point out to him that it's vital evidence in his favor.

He was nine years old and able to provide police with vital evidence.

He can't have any vital evidence - nothing that'd be in dispute, anyway.

Their investigation lasted two weeks, and that allowed vital evidence to be lost.

He did not mention the name of the young man involved and I said that might be vital evidence.

"But well lose vital evidence for a federal investigation."

Such notes can even become vital evidence if a malpractice suit is filed.