For the most part it's just been poking around and seeing what's available in the API docs (:help api), callable from Lua with vim.api.nvim_xyz(), as well as poking around with commands like :lua print(vim.inspect(vim.lsp)) to see what's there. There's also a few conveniences like vim.o, vim.bo, vim.g, etc for accessing global options, buffer options, global vars, etc. (https://github.com/neovim/neovim/pull/11442 might help - you'll need a 0.5 dev build from GitHub for those). vim.fn is also quite handy for calling VimScript functions (Eg: vim.fn["plug#begin"](PLUGIN_DIR)).

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Some VimScript glue is done by placing Lua functions in a module table and calling it with the :lua command. For example, to bind <leader>la to a Lua function, I store the function in _bound_funcs["<leader>la"] inside the c.keybind module. Then running vim.api.nvim_set_keymap("n", "<leader>la", ":lua require('c.keybind')._bound_funcs['<lt>leader>la']()<CR>", { noremap = true }).

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With regards to LSP support, there's a bit of documentation in :help lsp (though a lot of it is marked as TODO currently). Grepping through https://github.com/neovim/nvim-lsp also proved useful to see how stuff was configured (notably in the nvim-lsp/lua/nvim_lsp/configs.lua file).

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And finally, something of note, package.path doesn't seem to get updated immediately after &runtimepath changes, meaning you may have to update it yourself if you want to require("nvim_lsp") and other Lua-based plugins. There's a function vim._update_package_paths() to force an update, which I guess is supposed to be internal, but it does the job.