You think the other two players without a history of bad behavior who also ended up sitting out the whole session had fun?
Actually, the player who plays the paladin did have fun and was supportive of the scene. The party has had a history of marching into battle and sometimes, when the going gets tough, leaving him stuck in melee while they all turn invisible, retreat or engage in other actions of dubious tactical utility that place him exclusively in harm's way. He appreciated the irony of having the roles reversed for an episode and hang back while the other characters put their necks on the line.
He had in fact calculated that his ride skill was so good that he could have ridden into combat on his griffin with a comparable armour class to if he had put his armour on. Now, im my view, his conduct was inappropriate for a paladin but the player, at least according to his own account, gain some enjoyment from bagging a bunch of experience from things he didn't kill, given how much experience the other players had bagged from monsters he'd killed in the past.
The other thing is that our gaming sessions tend to be very social; plus, we model the combat using lego so those not in the combat action (and indeed those who were waiting to take their turn in the round) were able to have an enjoyable social evening.
As for the other player, he stayed behind after the game and apologized for sitting out the combat. I explained to him that there was no need for him to apologize and that the game was not about everyone always having to do the same thing. Nonetheless, he felt (correctly) that he'd made the wrong decision. He'd also failed to properly calculate his unarmoured movement rate and had based his decision, partly, on a miscalculation of how long it would take him to arrive at the battle.
The fact that they didn't play indicates there was a massive failure to communicate, there... It's your job as a DM to address things like that, instead of just sitting on your ass, smugly saying "I don't care, they brought it on themselves."
I entitled this thread "Am I Wrong?"; how is that smug?
Initially, I wasn't at all sure that I'd done the right thing but thanks to the support I've received from the majority of respondents, I'm increasingly sure I did make the right decision.
Another factor in my increasing certainty that I acted correctly is the quality of the arguments people have put forward to suggest I didn't act correctly. Most of these arguments are premised on an understanding of the rules that is incompatible with both me and the majority of my players.
The other arguments are:
Numion's:
My solution: Have the Dukes coming to his senses take the same time as the donning of the armours. At the same time the explosion happened to damage the entrances to the tower so that the victims could be loaded as quick inside.
PCs arrive about the same time at the scene. If there were a couple of the PCs tackling the vamps before the others got there, they'd get whats coming. I'm all for punishing PCs, not players
So, the alternative is to have things go pretty well the same way except that
(a) a spellcaster of unknown origin would show up out of the blue and severely damage the entrances to the tower;
(b) the duke would refuse to come right away and would somehow detain the sorceror who had come to seek his aid as well;
(c) the vampires would become more focused on killing the heroic characters and assault them until they either died or were forced to retreat, largely abandoning their original project of turning the city guard.
Even if these things happened, all that would change is that there would four characters out of the action instead of three and the bard, mage and rogue would be more likely to die trying to save others' lives.
mmu1 and Tsyr's:
I'd have most likely ruled that the time it'd take some of the characters to figure out what was going on, decide what to do, talk to the Duke, etc. would have been sufficient to put on the armor using the "don hastily" rules - if they insisted on taking enough time to get it on without suffering penalties for taking shortcuts, I'd have started the combat without them. After all, the explosion waking everyone up is just a contrived hole in the Vampires' otherwise perfect plan designed to let players get involved - if you're doing that, talking about sticking to the plan is kind of pointless...
There are problems here too:
(a) The time to don full plate is identical to the time it takes to don it hastily.
(b) As far as I understand it, you're suggesting that I tell people they cannot act during rounds of my choosing on the grounds that they're busy thinking; this seems like an insane way to treat people who come up with good tactical ideas: "That idea is too good. I'm going to paralyze you for two rounds because that's how long it would have taken you to think of it."
I can't imagine playing in a game (regardless of whether I was one of the "problem" players or not) in which the DM is satisfied to let some of his players sit there for hours instead of acting like an adult and taking 15 minutes to try to straighten things out.
What would you have suggested I say? Should I have said to the armoured tanks, "You're not allowed to sit this combat out"? Should I have said to the courageous and valiant characters, "You have to wait for the armoured guys while people die." Should I have cast a sleep spell over the whole city (except the characters' mansion) so the vampires no longer needed to complete their attack in a short time?