vsftpd supports custom welcome banners. By default the vsftpd configuration files are stored in /etc/vsftpd. The main config file is /etc/vsftpd/vsftpd.conf. In this file there are two ways to display a banner. The banner_file parameter will allow you to build nice spiffy banners with multiple lines and paragraphs even (ASCII pr0n if you roll like that):
banner_file=/etc/vsftpd/welcome.banner
Or for simple setups (most are), the ftpd_banner parameter lets you configure a single line welcome string for unauthenticated users. Make sure this doesn’t wrap to the next line or the daemon won’t start.
ftpd_banner=Welcome to krypted.com.
The daemon will need to get restarted once changed. The easiest way to do this is to use /etc/init.d/vsftpd:
sudo /etc/init.d/vsftpd restart