Segun sa akong nahibaloan ug nabasahan. (See book of C.S. Canonigo, 1999 reprint, English-Cebuano booklet, page 197)
Ang U is used before B,M,P,K,G,L,D,S,T & Y as in kubkub, sumsum, lumlum, etc. Another rule for U is it is always used in the first syllable of every word.
Ang O is used before NG, N as in bongbong, lonlon.
Ang E ug I are interchangeable but it is more Bisaya to use I than E like in Bisaya instead of Besaya, as in dili instead of deli, or kini instead of keni. It is however too boring to read or see all I in one word as in Biyirnis instead of Biyernes. I believe in the use of I and E, the user has wide discretion. If he is a wide Binisaya reader, he knows what is best.