Okay so you used composer in a terminal, not with webmin I'm assuming. Well regardless of any other issues which may exist, this is a primary one that should be solved first.
I would suggest blowing up the directory, trying again with composer, and pasting the exact error that occurs when you run through the install with composer. It won't both not give you an error, and not give you a vendor directory. To not have a vendor directory would mean the composer install would error out, and we need more information so best to start there.
