I just wrote a simple guide on how you can setup a productive elixir development environment in vim.
Its really easy, just a few steps. For completetion , go to def and other lsp features i wrote a simple plugin (https://github.com/amiralies/coc-elixir) for coc.nvim which is just a simple port of vscode elixir lsp plugin (it actually uses the language server which vscode plugin uses).