Fiendish Giant Ant - What Int?

[Infiniti2000]

(Before reading this, understand that I agree with Cabral, but I will pose some rhetorical questions and provide clearer proof of the answer.)

A Fiendish Ant is not a Vermin; it is a Magical Beast. Therefore, the Vermin mindless trait no longer applies.

Why don't those previous special qualities no longer apply? From the template: "A fiendish creature retains all the special qualities of the base creature and also gains the following." Note that vermin traits are in fact a special quality. I also think you're overlooking the line you quoted, which says: "...but otherwise the creature type is unchanged." So, the type changes but it doesn't? To understand this, we look at the rules for applying a template and see the following, with my emphasis:

Size and Type
Templates often change a creature’s type, and may change the creature’s size.

If a template changes the base creature’s type, the creature also acquires the augmented subtype unless the template description indicates otherwise. The augmented subtype is always paired with the creature’s original type. Unless a template indicates otherwise, the new creature has the traits of the new type but the features of the original type.

So, a fiendish giant ant goes from Vermin to Magical Beast (Augmented Vermin). Looking under the Vermin type, we find that Mindless is a trait, not a feature, and is therefore lost.

Sorry for the longwinded answer, but I thought it important and useful to go through the whole process.