USAGE: minimum number of vowels?

I know some languages have only three vowel sounds . . . well,
let me be more precise.  Some languages recognize only three
vowel qualities, although they might extend the system to six
phonemes via length contrasts.  These three vowels are
(almost) always (analzyed as) /a/, /i/, /u/.  Most languages
seem to have a triangular system with those three vowels at the
vertices, differing mainly in how many stops in between are
recognized as significant.

My question: do any languages recognize *fewer* than three vowel
qualities - two or even just one?    If so, which vowel(s) tend(s)
to be "missing"?

-Marcos

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