[akanevsky]

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.