VT100 incorporates VT52 escape sequences with colour extensions. ANSI refers to the use of ANSI escape codes to do the same thing, while VT-UTF8 is a Microsoft specification - which, as you guessed, passes UTF-8.
You may see VT100+, a second Microsoft specification for a colour Windows terminal, described at the same link, which also defines certain ...
I found what the problem was, the original sources.list is as follows:
# deb cdrom:[Debian GNU/Linux 10.0.0 _Buster_ - Official amd64 DVD Binary-1 20190706-10:24]/ buster contrib main
deb cdrom:[Debian GNU/Linux 10.0.0 _Buster_ - Official amd64 DVD Binary-1 20190706-10:24]/ buster contrib main
deb http://security.debian.org/debian-security buster/...
If you only added the plain URL, then your /etc/apt/sources.list is invalid.
An example sources.list for buster with the main, contrib and non-free repositories would be:
deb http://deb.debian.org/debian buster main contrib non-free
deb http://deb.debian.org/debian-security/ buster/updates main contrib non-free
deb http://deb.debian.org/debian buster-...