After trying all the suggestions given here and other threads like this one I still couldn't get apt_get to work on my Vagrant Ubuntu 16.04. In the end removing the file apt.conf from /etc/apt resolved the issue for me. My remaining files in that directory are:
vagrant@default:~$ ls -l /etc/apt/
total 68
drwxr-xr-x 2 root root 4096 Apr 15 12:20 apt.conf.d
drwxr-xr-x 2 root root 4096 May 21 2019 auth.conf.d
-rw-r--r-- 1 root root 53 Mar 19 19:50 preferences
drwxr-xr-x 2 root root 4096 Apr 14 2016 preferences.d
-rw-r--r-- 1 root root 3166 Sep 10 15:55 sources.list
drwxr-xr-x 2 root root 4096 Sep 10 16:04 sources.list.d
-rw-r--r-- 1 root root 17640 Mar 19 20:04 trusted.gpg
-rw-r--r-- 1 root root 17281 Mar 19 20:01 trusted.gpg~
drwxr-xr-x 2 root root 4096 Sep 10 13:45 trusted.gpg.d
