Pronunciation of unstressed vowels In San Duanmu's Standard Chinese phonology it goes over unstressed vowels and mentions that they change in unstressed syllables. e.g.

a, e, (a, ɤ) > ə  [报上 pau ʂaŋ > pau ʐəŋ]
ou > o [木头 muthou > muto]
ai > ɛ [脑袋 nau tai > nau dɛ]
ao (au) > ɔ [知道 ʈ͡ʂɻ tau >  ʈ͡ʂɻ dɔ or ʈ͡ʂɻ ɔ]

However, there wasn't much detail on when which unstressed syllables these occur and I have seen some other changes not mentioned or different to those above:

ei > i
ao > ɑ [多少 two ʂau > two ʂɑ or two ɑ ]

Are there any definitive guides on how these should be pronounced in colloquial speech? I'm sure it varies by region though so are there any patterns in Beijing dialect?
I would say this is actually a feature of Beijing mandarin. Vowel unstressing occurs when the syllable is 轻声 tone.
In the pronunciation of "model pronouncers" like news commentators, this phenomenon, along with even 轻声, is minimized.