Get vault root with Obsidian.nvim

Use the client vault_root() function

local client = require("obsidian").get_client()
vim.print(client:vault_root().filename)

See: obsidian.nvim client source

For getting a specific vault:

local client = require("obsidian").get_client({ dir = "/path/to/vault" })

For all the options available to the client, see the client class docs.