A key to understand the VMS

[Emma May Smith]

Suggesting that some symbols represent vowels assumes that the symbols represent either letters or sounds.
Both are natural assumptions, but I have very severe doubts about them.

The following symbols:  q  f  p  m  y   are demonstrably not to be identified with letters.
That's five out of (say) 25. How confident can one be that the others are?
And even if they are, what to make of a mixture of letters and non-letters?

I do not know why the characters you state are "not to be identified with letters".

I find absolutely no problem with both q and y being straightforward characters which represent phonemes. The characters f p m can be linked to other characters and have strong positional rules, suggestive of being variants.