......

blah.lua

return function()
  vim.api.nvim_exec([[ 
    echo luaeval("vim.fn.eval(\"'hi'\")")
  ]], false)
end

blah.vim

lua << EOF
vim.cmd [[
  call luaeval('vim.fn.execute("call v:lua.require(\"why.blah\")()", "")')
]]
EOF

this.md

Don't ask, just put this in your init directory like so:

lua/why/blah.lua lua/blah.vim lua/why.lua

Now run :lua require'why'. You're welcome.

why.lua

local fpath = vim.api.nvim_get_runtime_file('lua/blah.vim', false)[1]
vim.cmd([[ source ]] .. fpath)