It is completely supported to use Visual Studio Online and Xcode together, because both of them have support for Git.
The first thing you must do, after creating a Visual Studio Online account, is to create a new team project.
The important part here is to choose Git as the version control feature of the project.
The next step is to enable alternate authentication credentials. Xcode doesn’t allow user names that contains a ‘@’.
You do this in your profile, under Credentials.
Enter a secondary user name that does not include a ‘@’.
The third step is to clone or push the repository. You find the url to your projects Git repository under the code tab.
Go to Xcode and open the menu Source Control, choose the menu item that opens a sub menu where you can configure your source control settings. (See screen shot for a better explanation.)
Open the Remote pane and add your Visual Studio Online Git url.
Now you can push your local repository to Visual Studio Online. The first time you do this you need to enter the credentials that you created earlier.
I have had problem doing this the first time from Xcode. An alternative is to do it the first time from a command line. Follow the instructions above how to Push en existing repository.