Lerry the Lebster
All depends on who gets their stun off first
Ashe is reliant on a skillshot for her stun. If she misses with it, she dies. Instantly. That, and later in the game, Veigar can easily just ambush her and two-shot her.
Basically, as Lerry said, whoever stuns the other first wins. But Veigar can use his stun about 7 times more often than Ashe can with hers.
© 2015 Riot Games, Inc. All rights reserved. Riot Games, League of Legends and PvP.net are trademarks, services marks, or registered trademarks of Riot Games, Inc.