Open BUILD file in vsplit

mappings.lua

 function open_build_in_vsplit()
   local current_filepath = vim.fn.expand('%:p:h')
   local current_filename = vim.fn.expand('%:t')
   vim.api.nvim_command('vsp ' .. current_filepath .. "/BUILD")
   vim.api.nvim_command('/' .. current_filename)
 end

 vim.keymap.set('n', '<leader>vb', ':lua open_build_in_vsplit()<CR>')

Also see vim.opt.splitright = true

And the slightly more generalized command that drops you into ex mode so you can tabcomplete the filepath/name

vim.keymap.set('n', '<leader>e', ':vsp <C-R>=expand("%:p:h") . "/" <CR>')