Unless everything goes horribly wrong (and this being DF, it tends to do that), most ordinary threats (monsters, goblins, zombies, FBs etc) will only endanger those dwarves whose professions may take them out of the fortress. So miners, woodcutters, weavers, fisherdwarves and soldiers might expect high casualty rates, but normally you don't lose your best "inside" dwafves like weaponsmiths, farmers, cooks, brewers, strand extractors etc, unless you're unlucky or the fort is collapsing anyway.
Enter Urist McVampire.
Suddenly your legendary armoursmith winds up dead. Then you have to either put off that massive order of steel armour you were making, or accept it being of substandard quality. Either decision could lead to a disaster if your military meets something nasty in the meantime.
Or he might feed on your legendary cook. "That's OK," you say. "Quality doesn't matter too much on food anyway." You might notice when his replacement is cooking MUCH slower and you run out of food. Tantrum spiral may ensue.
In short, yes indirectly vampires can destroy a fort, though they seem fairly easy to spot most of the time. It is a bit of a shame that they don't resist arrest when the hammerer comes for them, but hopefully Toady will fix that in future. THAT could result in serious fun, especially if the vampire picks up a weapon, or was part of the fortress guard. Another reason not to draft new migrants straight into the army...