If you are doing two separate words like that without quotes, particularly those words that would show up in a huge number of posts, you are likely generating a gargantuan return set that is simply timing out before the database can return the results to the front end. Try using quotes, like: "VAC SYS"
I've found that using Google to search FOG yields better results than using the forum's built-in search. If you're using Chrome, you can search FOG directly from the URL bar in the browser.
Right click in the URL field, select "Manage Search Engines and Site Search" and add the following: