Specify the connection string to use with b:db or g:db.

let b:db = "mongodb://host/database"

Use :DB to execute the current buffer line using the mongo shell and show the results in a vertical split, takes a range.

:Mongo opens a buffer named './vim.mongo.js' with collection method autocompletion <c-x><c-u> and maps <c-j> to execute the current line :.DB in normal mode and the highlighted range in visual mode '<,'>:DB.