Mortars:
If you are a Streetfighter then dual Mortars will be great. They are also great with an Assassin but the way to Vanishing Strikes is more fun with a Streetfigher since he can use mortars/blunderbusses to great effect even before he gets to Power Level 9. The AoEs get backstab bonus dmg as long as the targets are not further away than 2m. Since CP Backstab is not related to the base dmg of the weapon but is an additional fixed raw dmg per attack roll, you can stand in the midst of enemies and AoE-Backstab a lot of them with the mortar shots. Powder Burns makes shure you also trigger "Heating Up" in order to get higher Sneak Attack and very short reloading times for the mortars. If you use Vanishing Strike as soon as you are bloodied you'll get even more out of it (because then you'll also have a lot higher crit damage). Since your INT is already very high the mortar's AoE size will also be very big which is a good synergy.
Here's what it looks like if you don't even use Gambit (which would be even better) but only Vanishing Strikes with dual mortars + Backstab + Community Patch as an Assassin:
Melee:
Sun & Moon: it's dual heads should grant two Backstabs per swing I reckon. It's also fast like all flails. It's great with Gambit because you'll have two chances to crit with only one swing (because dual head).
Sungrazer can be used with an Assassin + Vanishing Strikes to great effect. Extinction Event can trigger often because the Assassin gets +25 ACC for all his attacks under Vanishing Strike which leads to a lot of crits which naturally will kill some enemies. This will trigger Extinction Event which will profit from all the dmg bonuses like Sneak Attack, Assassinante and Deathblows, Legendary etc., too. It does enormous AoE damage then.
A setup with Sun & Moon + Sungrazer can work nicely with Full Attacks: Sun & Moon brings the enemy down and Sungrazer can already crit-kill. Also very good with Finishing Blow (it takes som time to get to Gambit after all).
Stalker's Patience will be good, Rust's Poignard is also good since both are accurate weapons and come Legendary. Stalker's Patience should be worn in the offhand because the recovery-skipping enchantment is of no use if you do Full Attacks like Gambit as long as the spear in in the main hand. Because the main hand's recovery get skipped anyway with Full Attacks like Gambit. This is a bit counterintuitive though because normally you would want the faster weapon (here: Poignard) in the offhand.
Like the flails both would not be my only pick though - because they only have one damage type (pierce). Actually a setup of Sun & Moon + Sungrazer in one weapon slot and Poignard + Stalker's Patience in the other would be good.
Scordeo's Edge is absolutely great because of Adaptive (ACC for gambit) and Blade Cascade (high benefical effect duration is also great here). You can't guarantee Blade Cascade though and often I find it to micromanagement-intense to look at my guy all the time if Blade Cascade already triggered or not and how long it's gonna last and so on. But of course it's supergood when under the effect of Vanishing Strikes. I would combine it with a Battle Axe + Bleeding Cuts because that's just top of the pops if you can skip recovery altogether.
Then you would have a good crushing setup (2 flails), piercing setup (stiletto + spear) and slashing setup (sabre + battle axe).
You can also do something like 1. setup with Scordeo's Edge + Sun & Moon, 2. setup with Scordeo's Edge + Rust's or so.
Other good weapon setups when using Vanishing Strike (+ Gambit) with CP Backstab:
Pukestabber: fast weapon, accurate, higher dmg, 20% higher action speed with all dagger attacks + Marux Amanth: fast weapon, accurate, +2 PEN at the start of combat for 20 sec base, chance of AoE attack. Use with Blackblade's Hood.
Rust's + Azure Blade: fast, good enchantments, Mob Rule grants +15 ACC with Azure Blade which is great and leads to more crits - but you have to be quite near to 3 allies! But summons/animal companions count afair. Also use Blackblade's Hood.
Keeper of the Flame + Xoti's Sickle with maxed Religion: the dmg bonus of the sickle can climb up to +60% iirc with high Religion - the speed bonus when standing near killed enemies is also great with high Religion. Keeper of the Flame does neat AoE damage with maxed Religion and you can get a small ACC boost - so why not?
There is no "best" weapon setup in general because it often depends on the enemies you are facing. For example gainst risen undead, skeletal warriors or flame blights Rust's would be useless while Sun & Moon would be great. Having all three damage types at your disposal is a good way to use the optimal tool against any given enemy. In some instances it may be even best to use Animancer's Blade or Eccea's Arcane Blaster (since raw damage). Underpenetration is such a severe malus to your dps that it's nearly always batter to switch ot another dmg type that can penetrate instead of clinging to on weapon with a supposedly op enchantment.
Against vessels you should use Grave Calling with Chilling Grave since it's the best against them. The Chillfogs it produces are foe-only and will get all the damage bonuses, too. THey will produce furhter Chillfogs on kill and get Backstabs.
And so on. You see there's no clear "best" in general.
Full Attacks with a ranged/melee mixed setup tend to be rather bad with Full Attacks because only the appropriate weapon for the current range will be used and not both weapons. Better have one melee and one ranged setup and switch if need be.
By the way: If you want to be even faster than naked you should wear an armor like Miscreant's Leather + a pet like Abraham or Cutthroat Cosmo. That combo of recovery reduction and armor penalty reduction leads to a net attack speed gain compared to being naked. Which means you get more attacks out in a given time.
Also make sure to wear Ajamuut's Stalking Cloak because Vanishing Strike allows to stun all enemies then with every attack which reduces deflection by 10 (more crits) and is great anyway. I believe that the Helm of the White Void will also give your attacks +10 ACC in this case because a stun effect is attached. It's a great headpiece to have as a rogue anyway (if you're not using it for a dedicated CC guy already).