Counting Vowels (coding assignment)

I need a little help with this assignement. I have to create an application that counts the number of vowels in a word of phrase entered by the user using the textbox. I got this assignement done with, but I had used the Case structure to accomplish this, but my teacher will not accept it.

Must haves in code:
For/Next loop
Chars(string method)
If/Then structure
LIKE comparison
a counter variable

Any help would be greatly appreciated, thanks!