inaudible (:)) vowels

which vowels tends to be lost in natlangs?
i've decided that in one of my conlang low central 'a' disapear, when
unstressed and consonant cluster is not hard to pronounce...

in polish, word 'w ogo'le' [vo'gule] ('-stress) is often said in rapid
speech as ['vogle]
in cree, short a, and i are missed in rapid speech
it seems for me that german langs lose sometimes @ - 'schwa'

is there any universal rule?
does some vowels more often disapear than other ones?

--
lucasso@friko6.onet.pl
http://www.lucasso.topnet.pl
(http://friko6.onet.pl/wa/lucasso)