Hi,
ldd `which xfce4-terminal` | grep vte
"libvte.so.9" => gtk2 (vte version 0.28)
"
libvte-2.90.so" => gtk3, but within that the older vte (up to 0.36, this bug not yet fixed)
"
libvte-2.91.so" => gtk3, newer vte (0.38 and newer, this bug should be fixed).
xfce4-terminal with gtk3 is only in git as far as I know, not yet released as a tarball. Not entirely sure, though.
Another, simpler way is to echo $VTE_VERSION, although it has the drawback that older vtes did not set it (and e.g. if you start xfce-terminal [using older vte] from gnome-terminal [using newer vte], it'll report the new version number as set by gnome-terminal).
As for xterm, Thomas will tell you what's up. However, I guess you can't expect anything rational if you misconfigure stty cols - just don't do it.