Vulkan Lives is generally poorly regarded.
Which makes it underrated.
Can't break these fetters
Alone, I staggered to my feet and saw a fallen son. It was Nemetor. Like an infant I cradled him, raising Dawnbringer aloft and crying out my anguish for all the good it would do. Because no matter how much you wish for it, the dead do not come back. Not really.
No book in this series exists in isolation, as made clear in plenty of Afterwords. Further, even if conceded that not much happens in Vulkan Lives (arguable) well, not much really happens in in the Horus Heresy. Most of the plot revolves around the feelings and interactions of demigods, the rest of it is just the canvas those feelings and slapfights are painted on.
Having that in mind:
(a) Vulkan has limited Horus Heresy lore, having taken an atomic blast to his face at Isstvan V; and
(b) if you want to rectify that, you can only make limited impact on the rest of the lore for, while there are the gaps, the main outline is set in stone and Vulkan's not in it.
Vulkan will always struggle to be the Shakespearean character the other Primarchs are in the Horus Heresy. The setting does not give Vulkan the "life" for him to be larger than.
So yes, Vulkan isn't especially memorable in Vulkan Lives. But I haven't read anything especially memorable about him elsewhere. His Primarch book was a dud. His limited short story appearances are dull.
As for Vulkan Lives, there is a praiseworthy economy of characterisation. The scene with the Eldar witchling makes its point. Vulkan's multiple deaths are a framing device for his chapters and contain interesting psychogical elements, rather than the gratious torture of Fulgrim in The Primarchs. I would even dare to hold that tortures such as strapping Vulkan into a murder machine is relatively subtle as Primarch characterisation goes, and I believe that "subtleness" has been misinterpreted as plainness.
While is antagonist Curze is partly a pantomime villain, Curze is also more coherent and driven than the weepy freak in other portrayals. He's an insane weirdo but he also a malicious dick... ...in a "cool" way, unlike Dorn. Dickishness is part of his flaw, and rubs against Vulkan's personality. I feel you can't credit the power of the final confrontation between the two without acknowledging the work beforehand, what Curze tries to take away from Vulkan.
Ok, there's a MacGuffin
‘A fair offer,’ said Pergellen. ‘Blades?’
‘Very well.’
Each legionary grabbed for his pistol, knowing that the other would do the same.
In addition to the clash between Vulkan and Curze, there's a B plot involving... ...look who really gives a s... ...pearpoint about the details, it's mainly some easygoing fluff.
Conceded, like Fear to Tread, the Space Marines in Vulkan Lives are not much more than workmanlike. But the Shattered Legions storyline(s) remain(s) a nice coda to the three Loyalist Legions destroyed at Isstvan V. As for their erstwhile ally, John Grammaticus is differently written here than in Legion, but that is par for the course for characters in this literary universe anyway. What I do really like is that there is a clear plot line, punctuated by a coherent twist that smoothly integrates with what exactly should happen (once you think about it). That twist is foreshadowed by the way the characters interact with each other, and a reminder that in Warhammer 40K, trust should not be lightly given.
‘You’re right,’ I conceded, holding up Dawnbringer so he could see it. ‘I fashioned it as a teleporter, a means to escape even a prison such as this. I counted on you leading me here, on you needing to face me one last time. It seems I was fooled into thinking you hadn’t planned for this.’ I lowered the weapon and let the weight of its head pull the haft down until my hand was wrapped around the very end of the grip. ‘But you’re forgetting one thing…’
Curze leaned in, as if eager to hear my words. He believed that he had me, that I would never escape his trap.
He was wrong.
‘What’s that, brother?’
‘It’s also a hammer.’
Vulkan Lives is not a "great" book, but it is underrated one. Perhaps it is fair to criticise the constraints under which it was written, however it is a very good book within those constraints.