So boiled down, you have to be the current target of the VWNM to be able to /fume, and 5 minutes must have passed since the battle started or the last /fume.

Also, a simpler option than waiting to fume is to spend the 1k gil each on Phase Displacers at the NPC behind the Jugner Outpost, and just use them on the mobs that use death/charm. Each one reduces the level of the NM by 5, and you can use up to 5 per battle. With 5 Trusts, that will make the bosses die really easily. Then you get the clear, and you can go back to proc item drops if needed.