The reason sudo vim opens vim is because vim on your system is set to open vim.basic through the Debian alternatives system. You override this for your personal user with an alias, but sudo does not use your user configuration, but the configuration of user root.

Rather than also setting up an alias for the root user, you better set up your system so vim points to nvim:

Execute the command

sudo update-alternatives --config vim

Select the option listing nvim (/usr/bin/nvim):

There are 2 choices for the alternative vim (providing /usr/bin/vim).



  Selection    Path                Priority   Status

------------------------------------------------------------

* 0            /usr/bin/vim.basic   30        auto mode

  1            /usr/bin/nvim        30        manual mode

  2            /usr/bin/vim.basic   30        manual mode



Press <enter> to keep the current choice[*], or type selection number: 

In this case, I would enter 2.

After this, you can remove your alias definition.