OpenVMS on VAX. There are plenty of terminal emulators out there that are 99% VT420 compatible. On *nix it's rare to notice that remaining 1%, but on VMS it's noticable.
Try xterm. Guy who develops it has a VT compatibility test suite, which most terminal emulators fail because they're mainly crappy xterm emulators. Xterm was/is written to actually emulate terminals.
In all seriousness though... sorry, friend - that's a negative re: xterm vt420 compatibility.
Quoting from the [current] author's own website at invisible-island.net/xterm/ :
"[xterm] also implements most of the control sequences for VT220, as well as selected features from other DEC terminals such as VT320, VT420, and VT520."
That "selected features" bit is why I keep the real glass one around. Paging is probably the most deficient area that I really notice.