Mmm interesting point. This means that either reflected sunlight is fine or that the loss of intensity makes it not not dangerous. I assume most people would agree that using mirrors to reflect sunlight on to a vampire is detrimental to their health and happiness. So it seems like a safe assumption that it is the intensity that matters.

Now we just have to find out if this is a property unique to the sun or if all star light at a certain intensity is unhealthy for vampires.

Time to build an interstellar space ship and fill it with vampires. Luckily they don't need much in the way of life support systems. So we can do this on the cheap.

I think I’ve read in some iterations, super old vampires have trouble from the reflected sunlight during full moons.