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 "...