Not sure I can add much here but last I used the breakpoint calculator, it had a ton of issues in general. What with not getting an answer to over half my questions I just haven’t bothered using it at all.
What I can say however:
As for the damage everything else here works about as advertised but I have no idea if or how Disrupt Destiny’s different damages (body/crit/weakspot) stack. Outside that I’ve made tons of tests and dmg calculations and it’s a bit tricky to describe how it all works in practice. Here’s a screenshot of the numbers I got from a BB test almost 6 months back (different talents & Scrier’s in general):
It’s been a while and something might have changed, but the point is I never did figure out why the dmg differs from expected the way it does. What I do know is that finesse boosters always apply to the portion of extra dmg from finesse hits. So if your weapon does 100 / 200 / 200 / 250 dmg (body / weakspot / crit / critWS), and you now add talents or perks for +30% weakspot dmg, then its real dmg will change to 100 / 230 / 200 / 295, respectively. CritWS hits get the full finesse buffs from both crit & WS hits, and they stack. So like here if critWS adds 150 over the base 100 dmg, and you had say +30% weakspot and +100% crit dmg, then the real critWS dmg would be WS = 150x0.3 (45), crit = 150x1.0 (150) → 250+45+150 = 445.
However I have no idea how that works for Disrupt Destiny’s different dmg stacks. Like if you have it at 15 stacks for +15% dmg, +37.5 weakspot dmg, +30% crit dmg, then do they all stack or apply to their own dmg columns only? Does the 100 / 200 / 200 / 250 become 115 / 237.5 / 230 / 351.25 (no stacking), or for example 115 / 267.5 / 260 / 388.75 (stacking), or is the stacking calculated in some other way?
tldr; The basic idea is that each bonus is applied additively, and almost all dmg bonuses apply to base dmg often even when not explicitly stated. But not always. But many blitzes, attack types, enemy armor types, even enemies themselves (like ragers with their x0.85 ranged dmg mod on headshots) have their own exceptions so the whole thing becomes a bit messy. And then ofc. there’s stuff like +Power which applies to total dmg after everything else, or debuffs like Skullcrusher which since applied on the enemy’s end means even that +Power is multiplied additively.
But it should go roughly like so: (base+all bonuses from perks & talents & blessings) → x Enemy Type perk → x Power → x Debuff → x enemy’s special modifiers +/-… iirc. 
I sometimes enjoy the Vraks VII on my psyker and deadly accurate is good on it, but I don’t bother putting headhunter on since you are guaranteed a crit every 5 shots anyways with the talent. Plus since it resets every crit, you aren’t ever going to hit the 5 stack bonus, which kind of makes it worthless.
You’re better off putting surgical or opening salvo on it. You don’t have to wait long for the opening salvo benefit or if you have those moments where you’re lining up a shot you get that extra time for surgical to build.
That said, Elodie is kind of right that assail already kind of does what you use the Vraks for. It has been a while since I’ve used mine when I run full right side tree. Las pistol, bolt pistol, columnus, or recon las fill in build gaps better in my experience.
Although Vraks can be a good gun psyker gun for those that want to use the middle or left side of the tree. For whatever reason.
HHs have kind of sucked since Patch 14, I wouldn’t recommend them over the mryiad of other ranged crit weapons available to Psyker atm unfortunately.
Maybe one day the MK 3 will have it’s unjust nerf rightfully reverted but until then it will stay on the shelf collecting dust.
Can anyone potentially shed light, here?