As Kirby mentioned somewhere, and as is mentioned in my
vBulletin API Tutorial:
Authentication Storage
The authentication data is stored in the following way:
$_COOKIE:
{cookiepfx}userid - plain(userid)
{cookiepfx}password - md5(md5(md5('PlaintextPassword'), salt), 'LicenseNo').
TABLE user:
password - md5(md5('PlaintextPassword'), salt)
Note that for cookie, {cookiepfx} is your board's cookie prefix. It is configurable via admincp and is accessible via the COOKIE_PREFIX constant.