vtem talks to a serial port or PTY, and reads in vt100 escape sequences. It then turns around and drives ncurses to make your terminal look like what it figures a vt100 terminal would look like.
vtem isn't as crazy as it sounds. First, it can let your xterm/urxvt/ansi/whatever terminal play with vt100 programs.
But it's also a nice firewall against malicious terminal sequences. If you've looked at the source to xterm, the phrase "vast attack surface" will come to mind. By vetting incoming sequences via a much smaller program, you can dial this back significantly.