He also uses Blighted Gloom, a very strong Dark elemental attack, which ignores shadows.
If this is directed at you, you can easily run out of range to avoid damage; it takes 10 seconds from the time he says "Mirror of Blighted Gloom...I command thee" before he launches the Blighted Gloom attack. Run away when he uses it. He can still chase you down during the 10 second warning period and successfully hit you with Blighted Gloom, so try to stay as far away as possible and allow your NPC to take hate off you.
If Your NPC is targeted, however, Blighted Gloom will be countered with Blessed Radiance (so Stalwart Shield works very well). Your NPC will take no damage from Blighted Gloom.
If you still have hate, standing behind the NPC (as you would do for Cover or Trick Attack) will cause the ability to hit the NPC instead. (See Talk Page for discussion.)
Blessed Radiance from a LV50 (uncapped) Stalwart Shield fellow will only take Vassago down about 5-6% doing approximately 200 damage.
He is reportedly very susceptible to Paralyze. Silence and Slow will also stick.
His melee attacks are very strong, dealing around 150-200 damage.
However, his melee attacks will hit for 100 and less to a Stalwart Shield fellow, who easily tanks him and self heals without you doing anything for 80% of the fight.
If 150-200 damage sounds like a lot to you, it may be a good idea to use a Tavnazian Taco for your food effect. MNK/WAR fully buffed with tavnazian taco never took more than 100 damage from Vassago, and Blighted Gloom hit for 119 damage. Counterstance is NOT recommended (we're talking over 500 damage if Blighted Gloom lands).
Since he's using a scythe, his attack speed is fairly slow.
If you or your Fellow is KO, you are ejected from the battlefield.
The NPC and Vassago will not do any action until you approach them, giving time to buff up.
You CAN'T use Sleeping Potions / Opo-Opo Necklace for TP gain (it will reset to 0%), nor can you use Reraise Hairpin before entering (it will wear off).