Okay, this is what I've got so far. Ya'll are welcome to suggest additions or changes. It's light on details, but the gist of everything is here, I think.
[X] Attempt to lure the Vetala out by acting incensed by the situation caused by Esha's "accursed" presence and the wrath of her father. If he needs her body so badly, we will use the threat of its destruction to draw him from hiding.
-[X] Ask Zaia if he can use his Alchemy to create a great deal of smoke to accompany a disproportionately small fire. If so, we'll put together a makeshift and apparently haphazard fire pit on the Marcella's deck using sand from the ship's fire buckets and bricks from the mess, in which we will begin preparations to burn Esha alive. The madder, more unhinged we act here, the better. Communicate largely in French or other non-native languages where possible to stymie eavesdroppers where we don't want to be overheard.
--[X] If Zaia cannot manage this with the supplies available, we will instead appear to arrange to hang Esha from one of the ship's spars, choking the life from her. This will require careful rope work to do without actually hanging the Dhampir, but a sailor of Antonio's caliber (+9 Profession: Sailor) should be able to rig up a concealed harness that will prevent Esha from truly being hung.
---[X] Make certain sand, sawdust, flour, etc/whatever is available, is strategically spread about the deck to make it more obvious if the Vetala attempts to reach Esha under the effects of an Invisibility spell.
-[X] Either way, we need the ship's rigging to be, er, rigged (I hate repeating myself like that, but it really fits here) to partially collapse at the flick of a rope (untying of a knot, whatever) so that an appropriate mass of sailcloth will drop down upon the Vetala when he makes his appearance. With three sails to choose from, hopefully he will be positioned properly to be temporarily buried under the falling mass for long enough to allow us and several of our men to dog pile and physically restrain him before he can wriggle or cut himself free of the trap. Once restrained, Roland, Inge, and Esha can use their combined physical and magical might to subdue the creature long enough for a proper sea burial to be carried out.
-[X] This gambit is risky and has a lot of moving parts, but the pressure of potentially losing access to Esha for force the Vetala to act hastily and without caution.
--[X] If we fail to trap the Vetala but still attract its attention and force it to attack, our archers will be ready to quickly dip their arrows in a jar of Holy Weapon Balm and Roland will attack with his own sword that has also been treated. Inge will target it with Snowball spells, Esha will use her Disrupt Undead cantrip, and Zaia will use his Acid flasks. All of our men will fight defensively, trying to keep the Vetala out of reach to avoid its deadly claws. If Inge has a Ripper whistle or some other way to call for his aid, he could also be very helpful.