We connected our two IBM 4381s (VM/CMS) to our BT mega/kilostream serial network using VT200 (and VT00 & VT52)-emulating terminals via an IBM 7171 protocol converter. I had the somewhat unenviable task of programming this beast, but it worked well enough, and (like all IBM gear) was very well documented. This would be mid-1980s. The 7171 was good because it meant we didn't have to replace all our terminals.