I experienced that problem before on my VP150 and I am using a Denon 3803. I shipped my receiver to Denon and they did not find any problem with the center channel but they said they upgraded my software.
Joe also helped out to determine if my VP150 tweeters are damaged, and we found no problem. Unless something is wrong with your VP100 you should be able to clearly hear the dialogues.
I subsequently found out that the dialogue from the center channel changes characteristics as you change the setting, i.e. widescreen will have different sound from prologic II, DTS-ES etc. Your receiver should automatically detect the source but if you manually change it this center channel sound degradation could happen.
Your problem might be different but this is something you could check.
