There is a prefix for the Conduit of Destructive Whispers amulet that says “100% Fire Damage converted to Vitality Damage to Fire Strike.”
Does this conversion works only for the skill Fire Strike, for all the skills in the Fire Strike skill chain, or for all Fire Damage while using Fire Strike?
The latter.
On a somewhat irrelevant tangent: Explosive strike is akin to nightfall in that they are both viewed as separate damage sources.
So you can think of explosive strike as an independent attack which occurs simultaneously the moment FS happens.
Therefore, any conversion affecting the FS line will not affect ES at all.
Ok, thanks for the reply, but I may need more clarifications! 
I get the thing with Explosive Strike, independent attack, so the Burn isn’t converted into Vitality Decay. But since Fire Strike has no duration, I guess it means that if I were to use, say, Blackwater Cocktail, it would still deal Fire and Burn damage, even if I were to spam Fire Strike during BWC animation, correct?
If that’s the case, I’ll have to see if the +flat Vitality on Fire Strike is enough to warrant a place for the COnduit in my Defiler build…
Firstly, you can’t spam FS during BWC’s animation. It’s binary.
Which is why most builds aren’t hybrid builds. Every second you spend casting another spell, is a second spent not using FS.
Secondly, even if you could, wouldn’t that conversion be the most broken thing in the game?
Lastly, the wording alone should give you all the clarification you need. 
100% X --> Y conversion to skill Z, not 100% X --> Y conversion to all skills used while using skill Z.
The exception to this rule of thumb is if this conversion applies to a ‘global skill’ like blood of dreeg. E.g. Darkblaze’s 100% acid converted to chaos to blood of dreeg - this affects all sources of acid damage.
EDIT: Last but not least, I would just like to give you the heads up that defilers suffer quite a fair bit because enemies stack loads of vit res. So a vit based defiler isn’t the best. Ditto for fire damage. Most skills using these damage types require double RR from both masteries to shine.
The only real exception I can think of is phantasmal blades, and that’s because of the insane damage it packs.
Of course, feel free to make the defiler, I just don’t want you to be blindsided by any nasty surprises
Thx for the answer, it is roughly what I suspected (even if you CAN spam FS during BWC animation, because this animation is really reaaaaaaally slow, you throw the bottle aaaaaaall the way, and it only explode when landing where you aimed…2 seconds ago )
As for my Defiler build, I don’t expect top results, nah, If I can farm SR 35 I’ll be satisfied. The core concept is a rifle Defiler. I have two main ideas :