Explosive runes as a weapon

[VorpalStare]

From the spell description of Dispel Magic:
You automatically succeed on your dispel check on any spell that you cast yourself.

Vraister

Maybe you missed part of the description?

From the description of Dispel Magic (PHB p. 223), next to last paragraph:

You may choose to automatically succeed on dispel checks against any spell that you have cast.

Emphasis mine.