Interesting... so what your saying is that the rules the way they are written say that you CAN scream into combat. I wasn't playing it that way the other day because I assumed it was considered it a 'shooting attack' (which is a weird thing in a vamp counts army).
I guess it makes some sense, i'll just have to think of it like the difference between magic missiles and direct damage spells.
Sorry, but I have read nothing indicating that it was intended to not work if someone was engaged.
Thanks for your help Dakka :-)