$ sudo vim /etc/fstab
It is easy to debug vim directly, but how would you do it when run with sudo? Well, here is one way to do it.
First, I will assume that you have vim executable file debug symbols. To find out how to compile vim with debug symbol, please refer to here. Say the file path is /usr/local/bin/vim.
Next, run the command that you want in one terminal:
$ sudo /usr/local/bin/vim /etc/fstab
Now, open up another terminal, and search for your vim process:
$ ps aux | grep vim
root 24687 0.0 0.0 63144 4212 pts/6 S+ 19:04 0:00 sudo /usr/local/bin/vim /etc/fstab
root 24688 0.0 0.0 34180 5684 pts/6 S+ 19:04 0:00 /usr/local/bin/vim /etc/fstab
linuxnme 24692 0.0 0.0 22572 988 pts/0 S+ 19:04 0:00 grep --color=auto vim
We see two processes by root, one is sudo and the other is /usr/local/bin/vim, which is precisely what we want to debug, and its pid is 24668. To attach gdb to this process,
$ sudo gdb /usr/local/bin/vim 24688 -q
Reading symbols from /usr/local/bin/vim...done.
Attaching to program: /usr/local/bin/vim, process 24688
Note that we are issuing sudo command before gdb here because we need superuser privilege to debug the process run by root.