So, I know there are certain consonants in the IPA that have vowel-like properties, and can therefor be used as vowels, such as [n], [m], and [l]. Examples include [pnt], or [ʒlf]. So, in the loosest possible sense, can a vowel ever be treated as a consonant? I realize that this is kind of a weird question, and the obvious answer is probably not, but surely, some language might have certain words that sometimes have a vowel in place of a consonant. Thanks!
1Did you forget about semivowels? In the loosest possible sense, those can certainly be described as vowels that are consonants. The following recent question and its answer are also relevant: linguistics.stackexchange.com/questions/17747/…– brass tacksJun 4, 2016 at 1:46
1Those consonants [n], [m], [l] can be used as syllable nuclei (ie they have the feature 'syllabic') in some languages but that doesn't make them vowels.– Gaston ÜmlautJun 4, 2016 at 2:10
A vowel has two closely related but inequivalent definitions. One of them looks what's happening in the mouth etc. Vowel is a sound in which the tongue doesn't touch anything and there's no build of pressure anywhere. That's why the sound may last uniformly and that's why it has the potential to be the peak of a syllable. The sound that allows a syllable to exist is the other definition of a vowel.
The consonants you mention – for example, in Czech, most famously, "L" and "R" – may form syllables, so they satisfy the syllabic definition of a vowel (my last name is "Motl" with an "L" and has two syllables; similarly "prcek" with "R"). However, they don't satisfy the tongue-and-pressure-based definition of a vowel so "L" and "R" are still considered consonants (although they are "syllabic nuclei").
On the other hand, vowels (by their sound) may fail to be the bulk of a syllable and they may be treated as consonants. These sounds are called "semivowels" and in English, "y" (in "yes") and "w" (in "west") are examples of such semivowels. By the essence of their sound, "y" and "w" are vowels. After all, "y" is a relative of "i" and "w" is the "double u". But because they're not the most important or long-lasting part of English syllables, they behave as consonants.
Vowels can't be consonants. Consonants can't be vowels. However, consonants can do things that are more commonly observed with vowels, and vice versa. "Vowel" is specifically a description of the vocal tract constriction of the segment combined with the prosodic property of being a syllable peak ("syllabic"). When a vocalic (vowel-like) segment is not syllabic (thus is more like a consonant, since consonants are typically non-syllabic), it is termed a glide (or semivowel).