The Vault: Complete list (Thanks to TSTOAddicts)

Wow. There are some really good buys on here.

Must buys if you don't have 'em:

Character-Building Combos
Akira w/ Happy Sumo (75 donuts*)
Krupt w/ Springfield YMCA (90 donuts*)
Jub-Jub w/ Pet Shop (65 donuts*)

Stand Alone Characters
Dr. Nick (65 donuts*)
Arnie Pye (65 donuts*)
Rich Texan (65 donuts*)
French Waiter (35 donuts*)

NPCs
Ha! Just kidding. There's no such thing as a "must buy" NPC.

Stand Alone Buildings
Aztec Theatre (60 donuts*)
Springfield Observatory (55 donuts*)
Shorty's (45 donuts*)
Vesuvius Pizza (35 donuts*)
Fireworks, Candy, & Puppies (45 donuts*)

Decorations
Jet Engine Bike (20 donuts*)

* - denotes net price after rebate

Honorable Mentions
Police Car, Ambulance, Knightboat, Old Gray Mare, Quimborghini, Sumatran Century Flower, Stacy's Dream House, Molloy & Museum