Available variables in each hook location?

[bigtime]

Is there a list of variables of each hook location? I searched but can't find it.

Specific question:

I'm trying to run this query in a plugin for the userdata_delete hook location. When a user is deleted from vbulletin, I also need to delete the same user from my review_users table.

$db->query_write("DELETE from review_users
WHERE username='$username'");

It doesn't work so apparently $username is not present as I have referenced it. What is the correct method to reference it?

Thanks,

Tim