Hah, the file
http://www.tuhs.org/Archive/PDP-11/Tools/Tapes/Vtserver/v7_standalone.tar.gz
has the source code for the standalone tools including boot and vtboot.pdp.
So, given a working V7 environment, you should be able to rebuild these
and possibly make them work in a V6 environment.
Alternatively, if you have a working V6 environment (e.g. a simulator),
you could bring in the vtclient.c code and integrate it into the V6
boot binary.
Cheers, Warren