If you try to execute 32-bit version of mstsc.exe in Vista 64-bit by passing a .rdp file as an argument, it will act as if you do not pass any argument. I experience this a lot since I use IE 32-bit to open .rdp files on the web which will redirect to 32-bit version of mstsc.exe. Nelly kindly answered me that it will be fixed in Vista SP1. I don't know how long I have to wait given that RTM version hasn't been available to retail yet
Bryant and Chris reported this issue a while ago. Now Microsoft just acknowledged the problem and put in a knowledge base article about this problem as KB911895. Hopefully Microsoft will release a resolution to this problem for Microsoft Vista Home Premium...