Less Then Optimal Party?

One of my golden rules as a Dungeon Master is that I want you to play the character you want to play in the game. As a player not only are you adding to the story and narrative of the game, but you are taking very-valuable real life time to play D&D- I want that time to be well spent.
So what happens when you have a less then optimal party setup?
D&D likes to have character assignments- usually something along the lines of fighter, wizard, cleric, and thief types. Maybe you can get away with a Paladin in some situations filling in as both the “fighter” and “cleric”.
But what happens if that next dungeon is really a tomb full of undead and vampires and the party is all fighters?  
Should you not run the adventure?
What if it is vital to your campaign, or just something that looks to be wicked fun?
Run it with some modification.
If there is obvious party imbalance through the classes, and they can’t find a work around, then in the example of our undead-tomb, maybe a local temple gifts them some anti-undead potions or gear.
Or maybe an NPC cleric.
It will still be a tough go, not having a dedicated PC slot, but in a nice-but-not-mean-way, that is the party problem.
Bring out your dead!