I read up on all the controversy of how animal guide is worded, but my question is much more simple... can they take improved familiar to improve the base familiar still?
I want to say yes but the animal guide never really dies and isn't really dismiss-able:
"If a magical child’s familiar dies, its consciousness simply leaves its body, and the magical child can replace its body after 24 hours."
so I don't think it can ever actually qualify for the "Ability to acquire a new familiar" pre-req. I could see a GM ruling that replacing its body could qualify, but still.. you know, words and stuff. :)
I would say no on the basis of not being allowed to take the Improved Familiar feat more than once. By the time you qualify for the Improved Familiar feat, you effectively already have it.
Admittedly, the text of the archetype does not actually say that you again Improved Familiar as a bonus feat, and my argument would not cover the corner case where you took only two levels of Vigilante and then advanced your familiar from there via such means as th Boon Companion feat.