Opponent moves Vex Cheerless into my battlefield and I reveal Hidden Blade from Hidden. Do I need to pay an additional cost?

Yes, you must pay an additional cost to play Hidden Blade from your Hidden state when Vex, Cheerless is in combat at that battlefield.

Explanation

According to RiftJudge FAQs #8518, #10610, and #7274, the interaction between Hidden and Vex, Cheerless follows this specific sequence:

1. Hidden (Base Cost): Playing a card from Hidden sets its base cost to 0, ignoring the card's printed energy and power costs.
2. Vex, Cheerless (Cost Increase): Vex’s passive ability adds a cost increase of 1 Energy and 1 Power to enemy spells. Because cost increases are applied after the base cost has been set to 0 by the Hidden keyword, this increase is added to the total.
3. Final Cost: The final cost to play your Hidden Blade is 1 Energy + 1 Power.

Important Nuance: Combat Requirement

This cost increase is only active while Vex, Cheerless is in combat at the battlefield. If Vex has just moved to your battlefield but combat has not officially begun (for example, while other "When I move" triggers are still resolving), the cost increase is not yet active, and you could play the card for 0 energy and 0 power. Once combat starts, you must pay the additional cost.