Mostly, they're used interchangeably by native speakers. I recommend searching the original text to see how they're used in context. key - not used in the tex

Mostly, they're used interchangeably by native speakers. I recommend searching the original text to see how they're used in context. key - not used in the text important - seems to be used when comparing things: more important or less important vital and crucial - used in the text, both of them seeming to indicate the highest priority