My friend Harry Mendryk over at the Simon & Kirby blog was able to give me a bit more information regarding the authorship of the first issue of Captain Marvel Adventures. While he didn't know if Manly Wade Wellman had worked on that issue, he did tell me that the stories were not written by Joe Simon. In court transcripts from the big 'Superman Vs Captain Marvel' lawsuit, Simon testified that he and Jack Kirby had asked if they could make changes to the scripts provided by Fawcett for Captain Marvel and the answer had been a firm no, because the Fawcett staff said they were following a specific pattern and that pattern was based on Superman. So no Joe Simon stories, despite what the credits in the Shazam Archives say. I think it probable that Bill Parker scripted at least one of the stories in CMA #1, as it refers back to an earlier Captain Marvel story written by Parker. My money is still on Wellman for the vampire tale though.
You can check out the excellent S&K Blog here: