As the game should never be played without copious amounts of terrain and some environmental rules, it is, truly, when it is an open field on a bright sunny day that Vulkans are at their best.
In one game I played, I had a squad of Vulkans in a subterranean game. At one point, they were holding an important passage intersection. PhillySniper sent his Soulslayers toward them. He failed his environmental roll - badly!
Suddenly, the entire intersection erupted with steam and smoke, effectively blinding my Vulkans. His Soulslayers made their saves - every last stinking one of them! The Vulkans fell to Soulslayers, even though they had superior firepower, night vision gear, and supporting troops.
As for the Vulkan Army of Doom, it is, to say the least, a nightmare of min/maxing, and is lacking in flavor and style. It is brute force, with just enough militia to field as many Vulkans as I can.
At last count, in a 1,000 PC battle, I could field 16 (yes, that's sixteen) of them. In a perfect world (clear LOS, 6-12"), that's 144 shots per turn, with, roughly, 70% of those hitting their targets. A quick calculation yields about 70 wounds in a turn. Most armies cannot, possibly, stand up to that. Hide, and get into CC with them. Reduce their LOS, hit them with flamers (radiant and residual) and hope that I roll poor dice.
The army no longer sees the battlefield. I merley threaten my opponents with the mere thought that I could bring it out...