Differences between expression and idiom, as well as colloquial and vernacular

Expression and idiom are used interchangeably, and so are colloquial and vernacular; albeit incorrectly. Please advise on differences in meaning and recommend a proper usage.

An expression is broader than an idiom. An idiom refers to a type of expression, and is usually not predictable by what it states e.g. Kick the bucket means "die".

An expression refers to the way we say something e.g. We say 'get up' to mean to get out of bed. It's not an idiom, but an expression.

Vernacular refers specifically to the native speech of a certain place or class of people. It's usually colloquial as well.

Colloquial refers to any language that is non-standard or informal.

So, vernacular is sometimes informal and colloquial, but colloquial is not necessarily vernacular, because colloquial can refer to slang as well, which is not vernacular.

Use idiom only when referring to an expression that is peculiar or characteristic to a language, an expression that has already been dubbed idiom.

Expression is used to refer to a particular way of phrasing an idea, and can include idioms.

Vernacular is used only specifically to refer to "dialects'.

Colloquial is used to refer to informal and non-standard in general

One more thought on vernacular. It has also assumed an extended meaning. In addition to regional speech it can also be categorized as the idiom of a particular trade or profession, which brings it closer in definition to jargon.

All those explanations seem to go around in circles to me. Why not just say that an idiom is a combination of words having a meaning that is not apparent from the commonly understood meaning(s) of the words in that combination? And stop right there? That way, an idiom can be found within any vernacular, within professional jargon, or within "proper" normal usage of the language. And no, "idiom" and "colloquialism" are not the same. Consider the sentence "The place where old Jake kicked the bucket is a fur piece down the road." You have an idiom ("kicked the bucket") and a colloquialism ("fur piece down the road"), but the meaning of the latter phrase can be discerned from the face of it, so it is not an idiom.