This is quite odd, I have not had any vampires die from sun yet, I have even turned about 8 sims in my sims family tree who I am currently not playing, and there have been times where they turn right outside near the pond in daylight and as soon as they turn they start burning and go home, I have had at least 5 vampires turn in the daylight under the sun that I am not playing and none of them have died, I never see vampires out during the day also, I only ever see them at night.
I guess if you were to personally invite them to an event as a group or have a club force them to be there, then they might have no choice but to be in the sun, unless you can convince everyone to get inside? I have yet to plan a social event, the only thing my sims have been going to are night based parties to eat all the dancers and events at night specifically to avoid daytime hours. The food is best found at clubs and stuff during the night. ;)
*Edit
It is also possible the game does not take into account that a sim is a vampire when choosing a sim with high skills to fill a certain task at an event, like how the game made my hired pianist a ghost at my last sims wedding.
