Written by I finally figured out (with some help) how to get Quicksilver to dial a phone number using my Vonage account and click2call. Implementation is as an applescript which gets passed the phone number to call as a text string, which it then munges into the URL which it invokes using curl. Script available for download here.
I have been thinking about using Vonage and have a Mac but hesitated because I wanted the click-2-call feature which they said was only available for PCs. Your Quicksilver fix will take care of this for you? Also, are you satisfied with Vonage itself as a phone option?