How many vowels are there in Brunei Malay?
The simple answer is: three. /i/, /a/, /u/. However, things are not quite so simple. Indeed, the Brunei Malay dictionary (published by the Dewan Bahasa dan Pustaka Brunei) shows prefixes with 'e' (indicating pronunciation as [ə]). As a result, while beli (to buy) is shown as bali in Brunei Malay, its prefixed form is shown as membali.
Is this right? If Brunei Malay only has three vowels, how can prefixes have [ə]? Shouldn't the prefixed form be mambali?
The problem is that the vowel in the prefix is often not as open as [a], especially among young speakers (perhaps influenced by Standard Malay, which has six vowels, including /ə/).
Here is a plot of the three vowels, measured from the reading of a short text by a young female speaker:
Note the substantial overlap between /a/ and /u/, much of which is caused by this prefix. If we plot the prefix separately, here shown as 'e', then the overlap is reduced:
However, in some cases the prefix is produced with a more open vowel, so there seems to be substantial variation.
For more information on the pronunciation of Brunei Malay and the chance to listen to the recordings on which the analysis was based, see here.