Posted this in feedback. Copy pasta it here as it kinda turned into a mini guide and not making a new topic cause no point to.
Figure I'll put 2c in after significant time playing psi (vessel) sin on sc and hc.
150 cognition isn't rewarding until super gear levels, character lvl 142+ and after obtaining shuriken flurry relic.
On hc I pushed to 142 to hit 150 cog but ended up swapping skill points back down to 125 cog plus endurance and tenacity. 150 cog felt like an overall downgrade. Very minor damage boost to shuriken flurry at the cost of a lot of toughness.
That being said skill points at charlvl 120+ don't feel like they add anything to the build.
Midgame (up to fauzt) is a bit of a struggle.
Full psi set doesn't really have synergy with the rest of the build. Orb of annihilation is nice for clear if you can sustain it, but that requires specifically building around it. Using it in combination with the rest of the psi skills is acceptable as long as you are careful with mana. Even then I was barely able to clear and survive vizjun (hc)and fauzt (hc) was a grind.
Build only comes alive after getting 2x sagittarius (2x ssu) and then feels comfortable after getting black masq (another ssu).
Gear has to wait for umo/effigy at that point as you can't really afford to use shop mo's on ssu's in the hopes of ramping damage up in the short term to obtain duplicate copies of gear to then perfect later on. Too costly.
1-5 fairly straight forward.
6 is first encounters with phoboss and it isn't too bad if you know to stack extra cold. Maybe you get lucky there for ssu umo's. If not try to go higher.
7 your only line of defense as psi sin takes a big hit. Avoid goes down by 30% and the map layout doesn't help.
7+ you're basically gambling on whether or not you get popped because your avoid gets tanked. You are also forced to balance range/melee as you must keep shadow refuge active to cap phys resist.
Kiting/stutter stepping with shuriken is mandatory to dodge projectiles and stay alive meaning you can't fully utilize your biggest dmg skill mind ripple. On top of that it doesn't charm in labs so you are on your own to survive. If you can stutter step away from enemies and still hit them with mind ripple at fastest cast breakpoint you might as well try to hit challenger in league of legends as ADC instead.
Get used to dying often unless you are wired on adderall caffeine and never lagspike. Die a ton if you only have 86fhr. Sometimes escape with 200fhr. Very specific itemization to hit 152 cast, 200 fhr and still try to deal damage.
Overall can't really suggest this build to a new or even average player as they will hit the midgame wall and probably give up.
Can't suggest this to anyone wanting to play solo self found/no trading.
I can only suggest this to someone that is specifically going for dual sagi vessel sin end game build and even then don't expect to faceroll anything.
Big investment, medium reward, is ok if you enjoy an active play style and don't mind dying a lot.