I found "New Management" unsalvageable. It simply isn't interesting. Besides, getting the key to a Zhentarim Inn is wholly unsuitable for most parties, I'd wager.
Instead, I drew from several sources (PotA, SKT, this thread, The Womford Bat PDF) to arrive at the following substitute scenario hook:
While senior sword
Chalaska Muruin generally prevents
the Womford Bat from feeding on Zhent guests and trading partners, sometimes the gains justify the risks. Muruin has learned one of the current traders have broken into her safe and copied sensitive information on the Black Network. This cannot be allowed to leave the compound!
So she closes down Bargewright Inn. Then her plan is to delay while the Womford Bat feeds on each trader, after intimidating the truth out of them. As each one disappears, she has a reason for going through their belongings (if the Bat doesn't find proof), confiscating anything valuable regardless of whether the trader was innocent or not. Ka-ching.
As long as she successfully roots out the culprit, she figures she can justify her actions about the others to the higher-ups.
The traders currently staying at Old Bargewright:
Arik Stillmarsh, thin pale human, rare books vendor
Yordramma, a red-robed “monk” female with a shaved head (Red Wizard of Thay) and her three apprentices
Let's assume she's the culprit here, but as the DM you might want to leave that up to chance. Or perhaps the twist is nobody's guilty...? Regardless, the scenario secret is - this isn't particularly vital. The scenario's real job is to freak out the PCs...
Saliel, tiefling too-wide smile, trafficker in pleasure drugs (and really demon truenames), summons up any bodyguards as needed
Kendrin Feldarr, hot-tempered dwarf, exotic animals dealer with 1d6 wranglers, accompanied by a tame guard Owlbear or something
The party
Innkeeper Nalaskur has not been informed about any of this, and might just do something that ruins Muruin's plans (in order to rise in the ranks himself).
Nell Flairgarten, local busybody and eyes for Stillmarsh.
Inglor Brathren, senior handyman and conspiracy kook ("game over, man, game over!").
Grendo, Nalaskur Thaelonds protege
Beggus, Zhentarim thug
Pinky, Zhentarim thug
Orphala Schmid, widow of the Bat's latest victim (over in Womford)
The scenario is basically this (assuming the PCs sit on their hands) - one of the traders (except Stillmarsh ;-) ) get eaten, paranoia ensues, "incompetent" Zhentarims, Yordramma starts a fight with Feldarr killing him, more paranoia, Yordramma casts Fireball setting the Inn on fire but disappears after that. Then everything returns to normal, the party is free to go.
The party needs to do at least one thing:
1) find out the truth and have evidence to back it up. Muruin backs down, assuming they can't prove her connection to the vampire. Otherwise she needs to go all-out, commanding every sword to kill the adventurers. (Confronting her in private is preferable)
2) kill the vampire
3) assassinate Muruin - this will give Stillwater cold feet (even colder than usual) and he takes a vacation, possibly never to return. Getting away with murdering the Senior Sword could be challenging, though.
4) improvising and bluffing. Some players love to rewrite the truth. After all, this is D&D: roll 20 on enough social checks, cast a Charm Person or two, and maybe this drama gets an entirely unexpected resolution...!
5) flee the compound, a mini-adventure all of itself if they want their wagon or heavy stuff with them. Remember, their valuables are impounded while this is ongoing
6) convince other traders to join up (rather difficult considering mutual distrust and alignment differences) either to break out, confront Muruin, or simply for defense (the Bat doesn't dare attack), until Muruin gives up, the Bat starts eating her staff or both
7) otherwise just sit it out. See below
Muruin does not suspect the party - the Womford Bat will never target them*.
But the players don't know this! Once three out of four traders are eliminated (it just so happens Stillwater is the last standing trader each time you DM this...) Muruin lifts the lock-down, having gotten what she wanted.
*) unless the DM feels particularly devious, of course. Remember, the Bat would attack only a single PC, so this would be very deadly indeed.
Clues: like 77IM said, be generous with clues. Have there be significant glances between Stillwater and Flairgarten, and between Muruin and Stillwater. Have the party notice Flairgarten was always around just before a disappearance. Allow the party to learn Stillwater has stayed at the inn for over a year, while the others have stayed a tenday or less. Inglor could always say "he gives me the creeps". Nosy PCs should be able to find out Muruin's Swords loots each disappeared trader. The party should be able to sneak out after learning about the Womford Bat and be able to interview peasants across the river. Innkeeper Nalaskur is an excellent source of the "I hear our senior sword lost some pretty vital intel the other day, no wonder she's so testy..." kind of gossip. Breaking into her office nets several reports where she tries to avoid blame for the loss of intelligence, all scrapped (written before coming up with the brilliant Bat idea), if not outright stolen loot (after people start dropping off she might pick especially valuable trinkets for herself)...
I'd limit the number of red herrings to angry and scared traders only, and possibly Innkeeper Nalaskur, who could plant false leads (if the party has it too easy) or clue them in (because he hopes to get rid of Muruin).
Remember, this is just a seed of a scenario. An outline. I'm sure there are logic flaws that needs ironing out.
Thanks to 77IM and Fred Love for good NPC ideas