Prime Vault

The Prime Vault, an archive outside of Tenno reach, houses items that have been retired from the game's reward tables. "An item enters the Prime Vault" means that players are no longer able to obtain new copies of that item from the game. Preexisting copies of the items will remain in players' inventories and can be traded.

Items in the Prime Vault can be reacquired in limited-time Prime Vault Packages which offer retired Prime items and Prime Access-exclusive accessories. Prime weapon and Warframe parts included in the package will also become available in the Orokin Void during the duration that the package is up for purchase.

Frost Prime Vault
The Frost Prime Vault was available from December 8, 2015, to January 5, 2016.

Warframes

 * Ember Prime
 * Frost Prime
 * Mag Prime

Weapons

 * Boar Prime
 * Dakra Prime
 * Glaive Prime
 * Latron Prime
 * Reaper Prime
 * Sicarus Prime

FAQ
Q: Will my inventory blueprints be removed too?!

A: Nope. Anything in your Inventory/Foundry after the removal will stay.

Q: Are these parts gone forever?!

A: Nope. Items in the Prime Vault have the chance to come back at a later date.

Q: Can I still trade these parts after the removal??

A: Yes you can!

Q: Do the Prime items come with Inventory slots, Orokin Catalysts, and Orokin Reactors?

A: Yes!

Q: What gear is exclusive to Prime Vault?

A: Prime Vault Program reintroduces a selection of Prime Access Gear Exclusives and will only be up for purchase included in the "Prime Vault Pack".

Such exclusives listed are: Frost Prime Avatar Pack (Regular Frost Avatar pack available for purchase in-game), Misa Prime Syandana, Titan Extractor Prime

Q: What gear is not exclusive to Prime Vault and will also be found in-game when the pack releases?

A: Warframes and weapons can be found inside the Void and crafted during the time a Vault opens.

Such items available listed are: Frost Prime (Warframe), Latron Prime (Weapon), Reaper Prime (Weapon)

For a full FAQ and other questions, please visit Warframe Prime Vault FAQ

Reference

 * Last Chance For Frost Prime Parts In The Void 3/24/15
 * Last Chance For Mag Prime Parts In The Void! 7/7/15
 * Last Chance For Ember Prime Parts In The Void! 10/6/15
 * Last Chance for Rhino Prime in the Void! 2/9/16