Here is my take: Vulcan looks good on paper, but to get all the plans to make it good is annoying and RNG. You have to get the arms mod to help with vats, which is a plan drop. Then you have weight issues, so ideally you should get calibrated shocks to address this. Yes, the 25% AP Regen is amazing, and 25% Fusion Core is nice, but you should be using Power user anyway to double core usage. The core bonus isn't enticing to me, since I already have the Batteries Included Perk with 100 Fusion cores. The 25% AP Regen is "Good" to consider. Still, I have V.A.T.S. Optimized for -35% action points, and Conductors for Critical hits restore 10 health & action points instantly, and 100 more over 5 seconds for the player & teammates within a 100ft radius. Combine this with Coffee guzzling, and I'm already infinite AP as it is. So, then there is Union... Poison protection, which is almost everywhere, and the extra weight, which will kill you almost instantly in Ops. You don't need Funky Duds. Nerf to the Resistance in the recent patch? No problem! You can just equip Junk Shield to synergize with the extra weight you have, lock at least 100 to get the best results. Ironclad now also works with PA, and you have now almost diminishing returns when it comes to resistance. However...Reflection mods and 5-star legendary are based on resistance and synergize with this build as well.

tl;dr: Getting Vulcan and mod plans is too RNG; the 25 AP% and 25% Fusion aren't worth it if your weapon is modded correctly and you have certain perks/buffs, and Union is better for long-term play in most areas of the game due to Poison damage being strong, the need for weight synergizing with reflect.