I've read that Pfizer has started its HSV mRNA vaccine trial this year. It's true that prior efforts have not been successful for unclear reasons. And I agree that HSV-1 vaccine (or better yet, pan-herpes vaccine) would likely be more effective than shingles vaccine in preventing dementia.

I'm aware of an RCT investigating the efficacy of valacyclovir for dementia, which started in 2017. It's expected completion is this year but there's no news yet, as far as I'm aware.