In VS2015, nuget 3 has been set as the defaut nuget version (it was 2.x on VS2013-). This 3.x version is still unavailable on VS2013 at the date of writing, but should be soon.
The nuget 3 version in VS2015 RTM has a bug: it does not popup the authentication dialog when accessing a protected nuget package. You can either wait for an updated nuget addon with the fix - which may come soon or not - or use the simple workaround below.
The workaround is to put the credentials in the nuget.config file of your windows user profile.
To do this open a command line in a folder containing nuget.exe, and execute:
nuget sources update -Name Vapolia -UserName yourEmail@domain.com -Password thePasswordYouReceivedByEmailMake sure you named the nuget server "vapolia" when you added it in Visual Studio (check the case).
Edit: you can also update your project's nuget.confg file, which may exist in the same folder the ".sln" (solution file) is. If it does not exist create it.
Edit: it should work, but it does not. VS still prompts the password box for every new version of the nuget package, and does not obey the keep password checkbox.
Hope this helps.
Happy SVG !