Jude,
vBullein uses a full text boolean search. Each unique word will have it's own list of posts, so the phrase "blue juice" (without the quotes) will return all posts with either or both words in it, though posts with both words will be rated higher.
If you want the exact phrase, you'll need the quotes around "blue juice" to distinguish between matching posts like "blue juice is the best valve oil ever" and "the blue monkey likes his juice".
Here is a list of search operators:
vBulletin FAQ Forums - View Single Post - vB search or MySQL Fulltext search?
Hope this helps, even if the reply is almost two months late.
