Vampires and sunlight

Okay so I'm building a medieval-like world for a role playing game I'm the Dungeon Master for. I've got a bit of a problem I was hoping some of you might be able to help me with. <br><br>One of my players is playing an evil vampire, who's the leader of one of the factions of monsters in the game, however I've just realized I don't know how vampires interact with sunlight in this setting. <br><br>Sunlight can't just kill them because the player has characters who worship them as a living god, and it doesn't make sense that people would do that if the vampire never leaves their castle. <br><br>However, if the vampire is able to exist in sunlight, then the main villain of my campaign can't be a vampire, because they'd be too hard for the players to kill. I want this character to be difficult for the players to kill, but not impossible. <br><br>The best thing I can think of is to say that the vampire is weakened in sunlight, but not enough to be killed by it. I think I want the amount of time it takes for the vampire to be killed by sunlight to be long enough that the players will never see it happen. For example, if the vampire were to spend 24 hours in direct sunlight, they'd be killed. <br><br>Again, I want the players to never see this. I don't want the players to know that sunlight is this weak point. I want this to be something that is primarily relevant to my world building, rather than my campaign. This is because later on in the campaign, there's a plot where I want this to be important. <br><br>The problem with having this be this vampires weak point is that the players will see it as a weak point if they learn that. If the players learn that sunlight weakens the vampire, they will exploit that in combat. I don't want them to have that knowledge. <br><br>So I'm trying to come up with reasons why the players will never learn that sunlight is a weak point for the vampire. I don't want to just make this an IC/OOC thing, where the players know but their characters don't. I want to make it so that the players aren't even given the knowledge that sunlight is a weak point. I want this to be a part of my worldbuilding only. <br><br>The first thing I thought of was to make the players' characters think that the vampire is allergic to garlic, rather than weak to sunlight. However, this isn't good enough. The players will still try to exploit this by putting the vampire in sunlight, because it seems legitimate to try. <br><br>I need to come up with ways to make the players' characters believe that trying to exploit sunlight as a weak point is a stupid thing to do.<br><br>But I don't know what. Has anyone got any ideas? <br><br>TLDR: My main villain is a vampire, but I don't want sunlight to kill him. Instead I want sunlight to weaken the vampire enough that it will eventually kill him, but not enough to be exploitable by the players when they inevitably try to use sunlight as a weak point. I want the players' characters to believe that trying to use sunlight as a weak point is a stupid thing to do, in such a way that they will never learn that it is a weak point. I don't want the players to know that it's a weakness that they could exploit if they were capable of keeping the vampire in sunlight for long enough.