> aString := 'squeak'.
> aCount := 1.
> aString do: [:element | element isVowel ifTrue:
> 	[aString at: aCount put: $-]. "Could aString be self?"
> 	aCount := aCount +1].

Solutions using collect: have been posted, these do the trick.

There is a problem in the preceding code that will not be apparent until 
you actually use it in a program.  This code is not only changing the 
string, but also the literal 'sqeak' that was stored with the code.  If 
this block were executed repeatedly, this could lead to some pretty 
obscure bugs.