Does Effect veiler negate Decode Talker's ability to negate effect veiler?
In this scenario you can chain Decode Talkers effect to Effect Veilers Activation to negate the activation. So the effect of Effect Veiler doesn't matter to Decode at this point.

The Different here is when an effect is activated and when it resolves and how a chain is build and resolved.
You can look on the wikia for informations

http://yugioh.wikia.com/wiki/Chain

Or in the official rulebook, which also comes which each Structure Deck on page 40

http://www.yugioh-card.com/uk/rulebook/Rulebook_v9_en.pdf