Does Vex make your opponent pay to activate a spell from hidden while in combat?

Ruling: Yes, Vex requires payment even when activating a spell from hidden during combat. Hidden only removes the base cost of the spell, but additional costs like those from Vex are added after.

Sequence:

• Hidden removes the base cost of the spell
• Additional costs (such as from Vex) are added after
• The opponent must pay these additional costs to activate the spell