Regarding the relevance of vowel sounds to rhymes, I really cannot understand why posters like EminemImsider are being crucified for being right. It seems that certain members on this board hate it when their online boyfriends/idols/virtual-dickheads-they've-met are made to look stupid, and thus act more foolishly in jumping to their aid.
I mean who is the biggest fool? The fool or the fool who follows them? (Ahem Kill You)
If you listen to Till I Collapse there is a very obvious case in the second verse which illustrates the importance of vowel sounds to rhymes.
Eminem pronounces TuPac as "too-parc" in order to rhyme it with Nas which is pronounced "nars"
Had Eminem pronounced TuPac as "too-pack" the rhyme would fail entirely.
Clearly Eminem is stretching the vowel sound, but the comparison is sound. Park does not rhyme perfectly with Nars, but it is clear that the two sound very similar because of the vowel sounds, and when they are placed in the verse there is nothing unnatural which affects the flow.