This information is based on information from the DEC VT220
Technical Manual. I've not actually tried this out myself.
TLDR: Yes, it's likely to work, and quite cheap to try using a
sub-$1 BNC to RCA adapter if you already have an RCA-RCA video cable.
Section 1.4 says that the "BNC connector for composite video output to
an optional slave monitor" uses "RS170-like" output, which is the
NTSC black-and-white television standard. Thus, the signal timing and
format should be compatible, as far as that goes.
There are three issues that might cause problems, most likely some
degradation of the display.
There's not general agreement on the voltage levels of RS-170;
signals range from 0.7 V peak-to-peak or even less
up to the 1.4 V peak-to-peak of the original standard. Given that
your system is monochrome or will have only a couple of shades of
grey, whatever voltage range VT-200 uses is unlikely likely to be
There's a warning in in that section of the manual: "the use of dc
coupling is not in strict agreement with RS170 specifications." I
don't think that this would be an issue either since it's only the
AC signal that carries the information; input issues might also be
fixed with a simple coupling capacitor.
As you pointed out, horizontal resolution may be an issue. As far
as horizontal lines are concerned this is purely an analogue
signal, so no harm can come from the TV not having enough
horizontal bandwidth to display well the full horizontal resolution
the monitor is using, but even 80-column text may be difficult to
read. Since this will vary for each television or monitor, the only
way to find out how good or bad it is is to test it with your
Rather than buying a whole cable there are BNC to RCA adapters are
available for less than a dollar from many sources. If you already own
an RCA-RCA video cable, sourcing one of these is likely to be cheaper.
(Note that your RCA-RCA cable must be a coaxial cable for video;
using a non-coaxial audio RCA-RCA cable is likely to hugely degrade
the signal quality.)