Vulkites are definitevely worth it, especially used in units of 10 as screens or to hold objectives. However, the best unit Fyreslayers have are the herthguard berzerkers, which I usually take above vulkites, especially in 1000 point games.
I've had some really good success using:
Runefather
Runesmiter
20 Hearthguard Berzerkers with poleaxes
10 Hearthguard Berzerkers with broadaxes
Runic Fyrewall or alternatively Zharrgron Flame Spitter
With all the Buffs going off you can get your big berzerker unit up to a 3++ rerolling 1's which makes them almost unkillable, while dishing out loads of damage
As mentioned above I use both, however the big units (20-30) usually should run the poleaxes since their sheer amount of attacks will dish out a lot of mortal wounds
hope that helps 😊