That's how it's written, but there have been way too many vaguely worded and poorly phrased sentences in this edition for me to be confident that's what they actually meant.
I'm not sure how I would personally rule this ability. On one hand, it definitely strengthens Counterspell if you don't even need to see the spell casting. On the other hand, it's Vecna and magic, so sensing it would A) make sense, and B) make it feel like Vecna is super powerful which is what he's supposed to be. Okay. Just writing that out helped me make the decision.