There's no such thing as "casting cost" anymore. It's an obsolete term which hasn't been in use for 10 years now, but still gets used in conversation. And when used, it gets used to refer to 2 separate, but related, things. There's Mana Cost and there's Converted Mana Cost.
Mana Cost is what's printed in the upper right hand corner of the card. It's what you have to pay to play the spell (under normal circumstances). Ancestral Vision has no Mana Cost. A nonexistent Mana Cost cannot be paid, which is why you can't play Ancestral Vision under normal circumstances. You need some way to play it without paying it's Mana Cost (Suspend, Mind's Desire, etc.).
Converted Mana Cost is a number derived by adding up all the Mana symbols in the Mana Cost. This is always a single whole number and all cards have a Converted Mana Cost. A card with a Mana Cost of
would have a Converted Mana Cost of 3. A card with a Mana Cost of
would have a Converted Mana Cost of 1 (notice the difference between those two). A card with no Mana Cost (lands, Ancestral Vision, etc.) has a Converted Mana Cost of 0.
Recoup cares about the card's Mana Cost, so gives it Flashback with a nonexistent Flashback cost, and a nonexistent Flashback cost cannot be paid any more than a nonexistent Mana Cost can. Chalice of the Void cares about the Converted Mana Cost, not the Mana Cost, so a Chalice with 0 counters on it will counter an Ancestral Vision, which has a Converted Mana Cost of 0.
