I've just tried this with an added 2.5ml of vinegar from the onion jar. It definitely works, marrying the (two!) onions to the drink beautifully.

Couldn't agree more :) If the vinegar from the jar is on the sweet side, a couple of drops of lemon juice helps balancing the sweetness.

Have not tried it but please be aware that not all Scandinavian akvavits are seasoned with caraway! Some are seasoned with dill, fennel and some obscure regional herbs. For an akvavit with a robust caraway aroma I would recommend Aalborg Taffel Akvavit.