I recently had problems with Windows Vista locking the local domain user account when it was connected to another domain's VPN for a period of time. 

This is a method to solve this issue:

1. Locate the .pbk file that contains the entry that you dial. To do so, click Start, type *.pbk in the Research Bar, and then press Enter.

             (This file is probably in C:\Users\<USERNAME>\AppData\Roaming\Microsoft\Network\Connections\Pbk)

2. Open the file in Notepad.

3. Locate the following entry: UseRasCredentials=1

4. Modify the entry to the following: UseRasCredentials=0

5. On the File menu, click Save, and the click Exit.