As we all know, the Vial of Poison card reads as follow:

"If you defeat a character in battle or psychic combat and cause him to lose a life, you may discard this potion and force him to lose an additional life."

During our last play session we had an argument regarding the definition "cause him to lose a life".

When I defeat my opponent and have to choose between 1) make him lose one life 2) claim an object from him or 3) take one of his gold coins, does the potion card in question trigger as soon as I just choose that he shall lose one life or does it trigger when he actually has to remove the life point? Becasue my opponent claimed, that since he had an armour and thus could potentially save himself from losing one life point, the potion did not take effect until the outcome of the die roll on the armour card had been cast.

When is the correct time to use Vial of Poison?

As we all know, the Vial of Poison card reads as follow:

"If you defeat a character in battle or psychic combat and cause him to lose a life, you may discard this potion and force him to lose an additional life."

During our last play session we had an argument regarding the definition "cause him to lose a life".

When I defeat my opponent and have to choose between 1) make him lose one life 2) claim an object from him or 3) take one of his gold coins, does the potion card in question trigger as soon as I just choose that he shall lose one life or does it trigger when he actually has to remove the life point? Becasue my opponent claimed, that since he had an armour and thus could potentially save himself from losing one life point, the potion did not take effect until the outcome of the die roll on the armour card had been cast.

When is the correct time to use Vial of Poison?

Your opponent claimed with reason on his side. There are two wordings in Talisman: "choose to take a life" and "cause to lose a life". The second one presumes that the opponent actually loses 1 life, so Armour and other life-saving effects can disrupt your plans. Vial of Poison should be used when the first life has been taken.

If you're not convinced check the FAQ about the Runesword.