VSFTPD - Change User Home Directory
You want to use the usermod command. You're going to want to use the -m and -d directives to modify their home directory and move its contents:
-d, --home HOME_DIR The users new login directory. If the -m option is given, the contents of the current home directory will be moved to the new home directory, which is created if it does not already exist. -m, --move-home Move the content of the users home directory to the new location. This option is only valid in combination with the -d (or --home) option.