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That depends on how you're asking it!
If you mean 'vague' as in the word itself, it is an adjective. According to Dictionary.com, their definition is "not clearly or explicitly stated or expressed."
If you mean 'vague' as in - what part of speech is broad or not thoroughly detailed, that would be articles. A commonly used article would be 'The'. The reason that this is the most vague part of speech is that there are very few articles and not many rules follow articles in comparison to the other parts of speech.
If you do not consider 'articles' one of the parts of speech in English, then the most vague part of speech would be interjections. Interjections are used to express emotions. Words such as 'Wow' or 'Yippee' are interjections. Also, filled pauses such as uh, er, or um are also interjections. Interjections are usually followed by exclamation marks. (Remember, almost any word can be made an interjection given enough emphasis.)
What else can I help you with?
What part of speech is What part of speech is?
adverb
What part of speech is without?
what part of speech is beneath
What part of speech is camping?
i want to know what part of speech is camping
What part of speech is forgotten?
The part of speech is a adjective
What part of speech is oppose?
Oppose is a verb.
What part of speech is the word re-clarify?
a verb
What part of speech is the word clarified?
a verb
What part of the speech is wicked?
Wicked is an adjective.
What part of speech on this diagram does the letter D represent?
The letter D represents a determiner on the diagram. Determiners are words that introduce and clarify nouns.
What is the part of speech and definition of manufacture?
part of speech
What is the part of speech of momentous?
The part of speech for this particular word is a noun.
What is the part of speech is answer?
The part of speech for "answer" is a noun.
What part of speech is What part of speech is?
adverb
What part of speech Without?
what part of speech is beneath
What is the part of speech for explicit?
The part of speech for "explicit" is an adjective.
What is the part of speech of twirl?
The part of speech for "twirl" is a verb.
What is the part of speech for unfamiliar?
The part of speech for "unfamiliar" is an adjective.
