You can use both NOT IN and NOT EXIST to generate the same results. The following examples show this. You can run them in the Pubs sample database.

SELECT * 
FROM titles
WHERE title_id NOT IN (
	SELECT title_id
	FROM sales )

SELECT *
FROM titles
WHERE NOT EXISTS (
	SELECT *
	FROM sales
	WHERE sales.title_id = titles.title_id)

I'm not sure why you're having problems. Are there NULLs in the vcRequestor or TolasKey columns maybe?

-Mike