I'm pretty sure that when we beta tested the VAX-11/780 in the summer of 1977 it had a DZ11.  We only had a few LA36 printing terminals on it so we never saw any issues with it.  Not so when I started work at DEC in 1980.  The VAX-11/780 I worked on had IIRC three DZ11a supporting 24 terminals.  The lack of DMA was a big issue.  We had to run our VT52s no faster than 300 baud or it brought the system to its knees.

-Paul W.