If I'm in combat with Vex, and they try to use Rebuke on me, can I use defy against it?

Yes, you can use Defy to counter Rebuke.

Here is the breakdown of the interaction:

1. Reaction Timing: Rebuke is an Action spell. When your opponent plays it, it goes onto the chain. Because Defy is a Reaction, you are allowed to play it in response to the Rebuke on the chain.
2. Cost Check: Defy checks the printed cost of the spell in the upper left corner of the card, not the actual cost paid. Even if Vex, Cheerless is in combat and increases the cost of your opponent's spells, Defy ignores that increase and only looks at the base printed cost of Rebuke (2 Energy, 2 Power). Since this is within the range of what Defy can counter, it is a legal target.

According to the rules regarding Defy (as noted in the FAQ), it always evaluates the printed cost of the card, and it can be used to counter any spell that meets those cost requirements, regardless of additional costs or modifiers applied during the casting process.