Really great presentation by Richard Thomson at VCF Midwest this year about How the DEC VT100 Terminal Was Secretly an 8-Bit Computer which includes thanks to vt100.net:
https://youtu.be/6Ec97GFiqWA
In addition, there was ECMA-35 as well as ECMA-48. ECMA-35 provides some important context to ECMA-48, and the reason that ECMA-48 appears to not concretely define some stuff is that actually that stuff is in ECMA-35.
vttest post-dated these standards by three quarters of a decade for the latter and by a dozen years for the former, and wasn't really a comprehensive test suite until the 1990s.