I don't know if there's a vBulletin formatting switch for this problem, but after 20 minutes of futzing I'm forced to conclude that it's certainly an annoying problem.
I tried to modify my signature to include a link. The "Edit Signature" utility in the User CP pops up a narrow little text box and starts wrapping the text at about 36 characters.
For a two-line signature limit this means that the link text, the link formatting, and any other formatting (text size, color, line separators) has to fit within 72 characters of the text box.
Yet when you look at a user's signature at the bottom of a post, the available horizontal space for a signature is at least 80 characters. For small fonts it's more like 100 characters. The text-entry utility is artificially restricting the two-line limit of a user's signature.
I don't know how this could be fixed. One option would be for vBulletin to give users a bigger text box to enter their two-line signature-- to match the size of the text box to the size of the box that displays their signature. Another option would be to only count the actual characters in a signature and not the formatting.
A very straightforward solution would be to raise the software limit on the number of lines in a signature so that we can get around the text box/formatting problem, while administratively asking users to limit their sigs to two lines.
Below is a copy of the offending text/formatting that I'd like displayed in my signature ("Quote" it to see the additional formating characters).
For more info see my profile...
And below this post is my current signature, the only permissibly-formatted way that I'm able to redirect a user: