The primarch of the Slamanders, Vulkan is a force to be reckoned with both in terms of the background materials, and in terms of the actual rules.
Firstly, he's an army force multiplier, supplying bonus leadership and adamantium will to all his sons in the same detachment. They actually probably don't really need this bonus due to the legion rules, but it certainly boosts it to a good degree.
Due to Blood of Fire, Vulkans staying power rivals Mortarions. He re-rolls all IWND and Deny the Witch rolls due to sheer bloody mindedness!
Naturally, his armour is amazing: 2+/3+ is strictly awesome even amongst primarchs, but on top of this, he halves the S of certain types of incoming fire against him. Eat your hearts out Iron Hands.
His hammer, Dawnbringer, is just amazing. S10 and AP1 in melee cannot be underestimated. He can take on anything (ANYTHING!) in the game and win with this, short of another primarch. Instead of using this in melee, he can instead use a special earth-shatter attack. This is lower S, but provides a blast marker instead which is nice for when Vulkan is in danger of being mobbed and pulled under from massed marine assaults.
For a ranged weapon, he has the Furnace's Heart - a beam like weapon given to him by Ferrus Manus. It has a reasonable range for primarchs at 18", but can certainly cause mass wounds due to the line of effect rule. At S6 and AP1, don't count this weapon out - ever.
Overall, I feel that Vulkan is one of the best primarchs for being a sheer beatstick. His army wide buffs are not strictly required, but simply a nice addition. Hence he's not doing anything for his force that (e.g.) Alpharius might be capable of. Hence I think he's more of a Angron style character in many ways, but different in temperament to say the least. Points wise, he's probably worth it in my opinion, largely due to his staying power.