I have exactly the same problem in a mixed Cisco-Tandberg + Codian + Polycom VSX/HDX enviromnent. I have not investigated deeply why this is a problem, but I suspect it may be because Polycom endpoints have a strange tendency to strip domains during h323 call setup?
I quicly found a work-around when we recently got our Lync 2010<>VCS test integration up, and have not yet bother to report a TAC case (would that be with Polycom or Cisco?....)
Work-around tip: Try to register a Polycom HDX with VCS using SIP. This works OK for me. Remember to change to the new sip uri in the findme user account settings, if applicable. The HDX's and some VSX's can do dual h323/sip stack, I guess for some V or VSX's you have to do eighter h323 or sip.
I hope this helps, but also that a better solution exist.