Might I suggest a slight redesign so that the “search by keyword” and “search by user name” don’t share the same search options, and hence, the same defaults? I suspect that a LOT of people attempt to do a search for their subject before posting a question, and don’t notice that they are only searching the last 5 days. That is a reasonable default for “user name”, since you probably want to see the recent stuff posted by a user, but not for the keyword search. You could put the two types of searches in two different sections with different search buttons, replicating the search option fields. You could change that by just diddling around with that page to have two forms, leaving the field names alone so that you didn’t have to change anything else.
You could also have the search type chosen by a radio button, and reset the defaults via javascript when the type was selected, but that’s more extensive, and I strongly suspect that you do not want to introduce javascript. I also don’t know how much tweaking you can do to the .php templates without consulting vBulletin.
You could also put a specific “any date” search on the “new thread” page, and encourage people to use it before completing their post.
And yes, I DID use the existing search function and found the article about the default being set lower for reasons of resource usage.