When I used this plugin for a first time and set up my account, it worked perfectly, but I'm not able to change account settings because of missing account management. In Account section I can see onlymy VK account name, and no buttons to change or add another account. Screenshot is attached.
Looks like there is a problem in kdelibs that makes impossible to clear the cookies used by KWebView. As long as the old cookies from the previous login operation are here, VK.com will automatically log you into the previously selected account.
See also this thread from the "kde-core-devel" mailing list: http://old.nabble.com/KIO---KWebView-and-PrivateBrowsing-%28Cookies%29-tt33734741r1.html
As a workaround you can run Konqueror, go to "Settings->Configure Konqueror->Cookies" and clear the cookies for all *.vk.com domains. After restarting VK KIPI plugin, it will ask for login/password again.