Where is vsftpd.conf?

You've a weird issue, try clearing you cache, update your repository information and install it again:

sudo apt-get clean
sudo apt-get update
sudo apt-get install --reinstall vsftpd

If this did not work, purge vsftpd (remove it including its configuration files) and reinstall it:

sudo apt-get purge vsftpd
sudo apt-get install vsftpd

It sounds like the installation didn't complete successfully. The quickest solution is probably going to be to reinstall vsftpd:

sudo apt-get install --reinstall vsftpd

Late to the party bu hope what I discovered is useful.

The file is not there after install. However

/etc/vstftd.conf

will be created after starting up the server.
It's installed with

sudo apt-get install vsftpd

and started with

sudo systemctl start vsftpd.service

This should do the job.