There is a much simpler way to achieve the upgrade:
1st goto -> Software Sources -> Updates
There is a field that allows the system to check for new versions:
LTS only / any / never. Select "any" LTS will only be available with 16.04.1 Just remember to set it back to LTs before Oct.2016 if it's not in your nature to go in sync with every new release of (K)Ubuntu.
Then open the update-manager from the menu. After you have checked for updates, it will inform you about the availibility of a system-upgrade.
It's always recommended to read the Release-Notes beforehand for "Known issues". You find them in 7. Further information. It may also be a good idea to look at the Ubuntu-RN as this gives the underlying system, like drivers.
In case you are not certain if your sources.list is messed up right now:
You can easily reset the sources-list to it's origin without touching it with editor, as this was identified mainly to be a cause of errors.
This will raise a GUI-window, with a reset-button in the left-bottom corner(+ other option). What does this button? It retrieves the most current list of standard-sources via Internet connection from Canonical.
As in the log-list you have offered the term vivid appears, you are requesting an impossible solution. Vivid is not on the standard servers as it is EOL (End of life cycle).