I'd say we shouldn't. The amulet might not be worth half of our wages in the end. It's quite clear not of dwarven make, of unknown age, unknown origin, unknown quality (to him), etc.
Instead, mooch dear mother: "Mother, I was hoping I could get one piece of jewelry from one of the traders but I don't have the money for it right now. Please? Pleaaaseeee? I'll even make some mana potions later for you as repayment.
Also, experiments in no particular order:
Somewhere, maybe after you go back to fishing, try extinguish vitality, similalry to how you drain it, but after you feel it, try to destroy it, or simply let it go uncontroled, instead of trying to bound and control it. It might be easier compared to actually draining it.
and
Find or make some moldy bread. Try to drain or extinguish vitality only in the mold. Try to make the bread good again. Eventually try to do the same with a sick tree, fish and etc again. See if you can deal with parasites this way.
And to finish something to consider:
If we get stuck in the vale, eventually start raising one tree and some random animals somewhere very isolated, with the excuse you are looking for other fishing holes. The animals you tell them to simply randomly "scout" the vale, carefully avoiding any elf or human. Eventually "find" the tree (it might be better if you raise the tree last and "find" it as soon as you raise it), and run as fast as you can to the nearest settlement, and tell that you found a very strange tree burning in a purple light (how could you possibly know it was necromancy after all?). When/if they identify it as necromancy, it's a good argument to the council unseal the vale, after all seems some necromancer already got it, and it might be good to have elves outside trying to gather information. With some luck the undead animals you raised will be occasionally found every now and then much after you left the vale and reduce suspicion of you.