If Vecna's feature was this:
I would agree with you completely that Vecna simply has to see the creature.
But as it stands, it is a reaction. A reaction to what? A creature that is casting a spell. If Subtle Spell is used and there are only V,S components, how does Vecna know a spell is being cast? Without knowing that, he has no reason to use his reaction to try to stop it. If he doesn't use his reaction, he isn't using the feature.
And FWIW, three opinions, regardless of how knowledgeable they appear to be (

) when
everyone I've spoken with agrees there is no difference in the language used between "that is" and "in the process of" in this instance. You have
your experts, I have
mine.
Unfortunately, with natural language, 5E is rife with such issues, such as Flurry of Blows:
View attachment 251628
Does the wording "
Immediately after you take the Attack action on your turn" mean:
1. You take the Attack action to make an attack, then you can initiate Flurry of Blows, then finish with Extra Attack, OR
2. You take the Attack action to make an attack, and an Extra Attack, and then you can initiate Flurry of Blows.
How important is the word "immediately" in this case? What about the phrase "after you take the Attack action"?
And what about moving between the attack action and beginning your Flurry? Can you move or does "immediately" mean you can't move? Is this a case of the specific "immediately" trumping the general rule you can move between attacks?
Does "after you take the Attack action" mean you have to complete it? Or once you make an attack, is that enough?
FWIW, JC seems content to rule immediately still allows you to move, but after you take the Attack action means you must finish Extra Attack before you can begin the Flurry.
To make his take stick, a better writing would be: "
After you finish taking the Attack action, you can spend 1 ki....". This way it is clear you must finish your Attack action (which includes any Extra Attack) and then spend ki to gain the bonus action. By removing the word immediately, you can also move before using your bonus action.
But IMO "immediately" means immediately--you can't move, but "after you take the Attack action" means you've attacked, it doesn't say "after you
finish your Attack action." After all, your bonus action is separate from your action, so is separate from your attack (usually?).
You might not agree (which is fine--my apologies for getting you so frustrated), but the fact remains 5E's use of natural languages leads to WAY TOO MUCH interpretation--even between experts.

A few key changes in the wording, and the intent becomes much clearer (see my rewrite of Dread Interruption above... IF that was their intent at least).