GeanyVC is just as lightweight as the Geany itself :)
So in editor context GeanyVC rather assumes that the underlying repositories have been properly set up, which includes logins credentials needed for clone, pull (if any). This way GeanyVC does not need to deal with managing the logins for all the VCS it supports, and just focus on managing local file changes.
Some VCSs make it simpler to manages login details (like Git's remotes). Subversion has its own way of "remembering" passwords. SVN+SSH further helps make the authentication transparent by using proper keys automatically so you won't need to type either username or password details with SVN commands at all.
If it's still relevant, you may check out the SO thread [https://stackoverflow.com/questions/2899209/how-to-save-password-when-using-...) on the subject.