To unlock Doll, get Nephi after visiting the entrance of Ispolis, the go back to the Corridor Of Clouse. Go to the entrance of Abraxus' Tower, but do not enter. Instead, go to the right, examine the door, and Nephi will unlock it. Find the purple person, and examine him to unlock Doll.
To unlock Vai, collect the five Armaments after the world is fused and then return to Isapolis.
The Sigil pieces are located at the following areas:
Sigil Armlet - At the far southeast of the desert in the Black Send sub-area; there are stairs leading down into a secret area. You must defeat a monster.
Sigil Armor - Take the airship to the northeast area of the map. Enter the city, and fight in the tournament. Obtained after several fighters.
Sigil Blade - At the Draken Mansion, during the sub-quest with Kairu.
Sigil Helm - Purchased for 100,000 gold at or near the clothier in Tradefair.
Sigil Necklace - At the Cytherian Bridge, on the way to Tradefair.
To easily gain gold, after you obtain Catalogues 2 and 3, go to the unmarked island on the southeast corner of the world map. Buy a few Eternias from Catalogue 2 from the owner for 30,000 gold. Exit out, then talk to the owner again. Sell those Eternias to him through Catalogue 3 for 750,000 gold. Repeat as desired.