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