I don't understand what you've written. You seem to be putting 3 letters into 2 slots.

Assuming the slots are 1-5 there are 2 even positions, 2 and 4.

There are only 2 ways to put the "vocals letters" (a better English word is vowels) into these slots. I=2, A=4; or A=2, I=4.

Then there are 3! ways of arranging the consonants (the remaining letters).

2*3! = 12

so there are 12 arrangements that have vowels in even numbered slots.