You cant read a cookie of domain X in domain Y. This neither vb nor PHP related. This is cookie rule fixed in user side therefore you cant pass it.
And if you think about it it's wise and nesessary. Otherwise admins could be able to get passwords and personal data of their visitors in other web sites. For example would you like me to get your vb.org cookie (thus your username and MD5ed password) when you visit my personal site?