When does a vampire spawn decide to grapple instead of dealing damage?

Question

The vampire spawn's claw attack reads:

Instead of dealing damage, the vampire can grapple the target (escape DC 13)

When exactly is this decision made? In particular, I am interested in the interaction with the Drunken Master's Redirect Attack:

When a creature misses you with a melee attack roll, you can spend 1 ki point as a reaction to cause that attack to hit one creature of your choice, other than the attacker, that you can see within 5 feet of you.

Can the vampire spawn choose to simply grapple to avoid hurting an ally after the attack has been redirected?

Answer 1

RAW, the vampire spawn can choose to grapple after the attack is redirected

The vampire spawn can choose to not do damage and grapple its ally instead if its attack is redirected.

The vampire spawn Claw attack description suggests it can decide whether to cause damage or grapple after the hit (therefore after the Redirect ability says "cause that attack to hit one creature of your choice").

Caveat

However, as a DM I would say there is an argument to be made that the vampire spawn must decide its intent - damage or grapple - before it actually makes the attack roll.