On spelling Vietnamese there is one more language hack I wanted to mention:
since ơ and â are basically longer in shorter versions of the same vowel and so are i and y you will only find ơ and i paired together and â and y paired together. Long plus short and short plus long. Remember that y basically is a long i and makes the preceding vowel sound shorter so it wouldn’t make sense to put a ơ before y since it would sound like â. The same goes for i, it sounds different from y by having a longer sound before it. There may be exceptions. I’ve never read that this is a hard and fast rule. But there doesn’t seem to be any words in my dictionary that contradict this theory.

So if you have a word that sounds like schwa-eee then just try to figure out if it’s a long or short eee sound, i or y, and you know if it’s ơi or ây.

Another thing to remember about diphthongs in Vietnamese is that ia/ya, ưa, ua are at the end of a word but are spelled iê (also spelled yê), ươ, uô when followed by a consonant. Also remember that in diphthongs and triphthongs the vowels are not always pronounced exactly the same as if the vowels were alone.