Prowse DID read all Vader's lines on set. In the 4th draft script of Star Wars (easily googled), Vader's line is exactly as James Earl Jones reads it. It's possible David Prowse read the same line on set but maybe paused or read it more slowly and Lucas decided JEJ should read it the way that sounded best rather than reproduce Prowse's reading. Because Tarkin is standing still in the next shot, the editor had to stay on the wide shot with Vader gesturing until Tarkin stops walking. To me it looks like the editor just didn't have enough takes to choose from.