In the end, out of 40 NPCs and 7 PCs, only 3 PCs and 3 NPCs survived. The first casualty was a PC wandering the city at night. (The inhabitants welcomed the group, not sure what to think of them, but since the vampire 'gods' did not seem to mind them, the citizens were okay with their presence). He was captain of one of the two ships, and when a female vampire (hiding her nature) approached him in the middle of the city, he didn't mind. They started getting intimate, and she drained him dry right there, turning him into a vampire under her control. The vampire 'gods' wanted access to one of their ships, so they could learn how to make more, and wanted the crew.
The second casualty was a paladin knight. He had already fallen from grace partially back in the Old World by breaking his vow of chastity, and was hoping to redeem himself here. The vampire 'goddess' took an interest in him, and did everything in her power to seduce him. She succeeded. He was still loyal to his faith, but he could not resist her, or her minions for very long. He tried to protect his group, and refused to reveal anything which would endanger them, but besides that, she owned him. At the end of the adventure, he had been left behind, and is slowly being shaped into a dark knight for the vampires. (He has not been turned into one, however).
The third casualty was the most interesting, I think. And here's where I have a little bit of a conundrum. One of the PCs is from off-realm, coming from a dying world. One other PC is capable of making gateways to anywhere she has a sympathetic link to. The first PC gave up one of his weapons, crafted in his home city from that dying world, and the mage crafted a gate to it. The vampire goddess was being run by another player, and we discussed what she might do if they made a gate to the Old World. Her response was 'the goddess wouldn't go through -- she'd send a double'. The vampire god, however, was intrigued, and decided he'd investigate (run by me).
So, the PCs have this gate, and try to sucker the vampires through. The vampires are highly more powerful, intelligent, etc, than the PCs, and the PC doing the talking failed his Presence check to see if he's convinced the male vampire. Still, the vampire goes along with it, and checks out the gate. Before he'd go through however, he insisted one of the PCs (one that kept talking out at him) go through first. The PCs were all 'errr', but allowed it. The female vampire was there (well, her double was), and once the male vampire (her husband) had gone through after the first PC, she tried to get a second to go through. The PCs hemmed and hawed, and mostly worked around it, so she went to go through, and at the last moment tried to snatch a PC and drag him through. Failed.
Now, on the other side of the gate is a god-slayer. This creature destroyed anything which it senses as 'powerful' or 'god-like'. These male vampire registered as such, so the god-slayer came in and nuked the place. The PC's home city, the island, the sacrificial PC, the vampire, the 'dupe', and everything there, was destroyed utterly. No chance of survival. The PCs were notably sad at the thought of losing their comrade and the PC who came up with the plane was crushed at the knowledge he'd wiped out his own nation to kill two vampires, and it was a good scene.
Now... later on, they found out the vampire goddess had survived. Two of the players were ticked off at this, thinking it was GM fiat. They hadn't thought there would be a body-double, and didn't think to check for such. They had no real way of knowing, either. I eventually explained the situation to them (they had thought she'd just reformed back in this plane), but one of them remained really upset - this ruined the entire adventure for him.
In the end, the vampiress gave them the option of departing, but they didn't go through with it, instead attempting to lift the 'curse of darkness', and try to defeat her. In response, she used necromancy, and destroyed their second ship, then wiped out most of the survivors. There was a dramatic scene of them hiding in a temple, using guns and swords to fight off her warriors, while their mage frantically crafted a temporary gate home. The gate was made, the vampiress cast a 'curse of rot' which killed the surviving crew and almost killed the mage, and the survivors got home.
Really, the end fight was dramatic, only one player wasn't happy about it. The others, I can assume, didn't like the fact the vampiress outsmarted them after that crucial sacrifice, but the rest of the adventure, even if it ended as a 'loss', was pretty dramatic, and I think most enjoyed it for what it was.
The PCs had originally had a decent chance of success -- except that the 'unhappy' PC had back-stabbed the entire group, trying to get in good with the vampires, so he could betray them later. If it wasn't for that, and if the PCs had originally gone for the McGuffin they had a vision of early (instead of long after it was too late -- the paladin recovered it for the vampiress), they'd have had a much better chance of success, and may have liberated the city. As it was, they kept delaying, splitting up, openly defying the 'gods', etc. They shot themselves in the foot, often.
So, a few things:
1) Should the Vampiress have gone through the gate - even though she was suspicious, and had the mental attribute to plan ahead? Even though the PC had failed their social check to con the vamps?
ME: I don't think so. Sure, the PCs were... displeased... with being outsmarted, but I feel that intelligent NPCs should be played just like intelligent PCs. Back-up plans, and doing the smart thing.
2) Even though the PCs shot themselves in the foot, should I have given them a better chance of survival? Should I have the vampiress throw her best 'I kill you' spells at the end, after they'd defeated her husband, cutting off their escape routes as best she can, etc?
ME: I think so, since they proved to be a threat, and defied her offer of peace.
I don't know, what would you perhaps have done differently?