This is funny, but I think stereotypical vampire lore still stands. I think it’s only direct sunlight that is an issue. If a vampire stands in a room with an open window, but off to the side, they’re generally considered fine. If light reflected off a wall is ok then why is the moon not?
I actually agree, that could be pretty fun. That said this puts a lot of work / pressure on the DM so it is definitely something to be discussed rather than used to be lazy. (To be clear, I am not reading this into what you said. Just saying it out loud is all)