A recent advertisement from the August 7th Portland, OR stop of the 2015 Vans Warped Tourhinted a few more possible artists who have yet to be announced. Those artists in question include:

Attila
Memphis May Fire
Miss May I
Never Shout Never
Pierce The Veil
We Came As Romans
Beartooth
Senses Fail
Drama Club
False Puppet
Julie Vera
Knuckle Puck
Svetlanas

Prior to this leak, the newest additions include:
Silverstein
Mallory Knox
Man Overboard
Citizen
Carousel Kings (07/14-08/08)

Artists announced to date for the festival include:
Asking Alexandria
Black Veil Brides
Blessthefall
Hundredth
I Killed The Prom Queen
’68
Sirens And Sailors (07/14-08/08)
Slaves (07-14-08/08)
The Amity Affliction
Neck Deep
Palisades
Fit For A King
Crossfaith
Rotting Out
Escape The Fate
Being As An Ocean
Emarosa (06/19-07/12)
New Years Day
Set It Off
Kenneths
MC Lars
The Karma Killers
Mod Sun
Bebe Rexha
PUP (07/01-07/29)
Youth In Revolt
Juliet Sims
Koo Koo Kanga Roo
Metro Station
Night Argent (06/20-06/28, 08/01-08/08)
Riff Raff
Candy Hearts
BoyMeetsWorld
Ho99o9
Major League
Motion City Soundtrack (06/19-06/21)
While She Sleeps
Hands Like Houses
Le Castle Vania
Have Mercy
Our Last Night
ChrisB
PVRIS
Alvarez Kings
Moose Blood
New Beat Fund
Jule Vera
Seaway
This Wild Life
The Dirty Nil
Born Cages
The Wonder Years
Icon For Hire
Baby Baby
Matchbook Romance (06/19-07/11)
The Relapse Symphony (06/19-07/12)
Black Boots
M4Sonic
As It Is
Night Riots
Transit
Beautiful Bodies
Splitbreed
Handguns
Kosha Dillz
Alive Like Me
Family Force 5
Lee Corey Oswald
Trophy Eyes
‘Acoustic Basement Stage‘:
Aaron West and The Roaring Twenties
American Opera
Brian Marquis
Buttons
Grey Gordon
Koji
Meghann Wright
Onward, etc.
PVRIS (dates TBD)
SayWeCanFly
Speak Low If You Speak Love (07/09-07/14)
Transit