Dug up this: JR on CC, 2015: "The Dolby Digital AC3 Pro encoder is shipped as part of DVD Architect Pro and does not come with Vegas Pro at all. I've always had both installed but now that you mention it, I do remember that computers where I only installed Vegas Pro installed did not have the AC3 Pro encoder."
May be a trial limitation or indeed a licensing issue, if you have DVD A Pro already installed. The Vegas Pro (not Pro Edit) purchase should come with DVD A Pro. Might be that it needs to be a rebranded version?