The following states that I should have a my_vim.nix file (I’m assuming in my home directory?), but it doesn’t state what should be in that file.
https://nixos.wiki/wiki/Vim#Python_3_support_for_vim
Additionally, does the code there go in configuration.nix.
Trying to find where to put everything and I’m not sure where to start because the official Nix docs didn’t answer my questions.
That’s definitely not official docs in any sense. That unofficial wiki is dead, prefer wiki.nixos.org (I also suggest you edit your link accordingly).
Also that page is probably absurdly outdated, vim_configurable doesn’t exist, and python2 hasn’t been the default for anything in nixpkgs for a long while.
Simply: use pkgs.vim-full instead of pkgs.vim.
EDIT: you may also want to set wrapPythonDrv, i.e. (pkgs.vim-full.override { wrapPythonDrv = true; })