If Allay is on a bf with a Vex with deflect 1, to target the Vex is it one or two deflect pay cost?

To target a unit of Vex, Cheerless while an Allay, Eager Admirer is at the same battlefield, you must pay 2 Power as a mandatory additional cost.

Reasoning

1. Granting Deflect: Allay, Eager Admirer has the ability: "While I'm at a battlefield, your other units here have [Deflect]." When Vex, Cheerless is at the same battlefield, she is granted the Deflect keyword.
2. Deflect Value: Per Rule 809.1.b.3, if the value is omitted in a Deflect keyword, it is presumed to be 1. Both Vex's base Deflect and the Deflect granted by Allay have a value of 1.
3. Summing Deflect Values: According to Rule 809.2: "If a Unit has Deflect... and is granted Deflect by an additional source, the Deflect Value of all granted Deflect keywords is summed." Vex (Deflect 1) + Allay (granted Deflect 1) = Deflect 2.
4. Additional Cost: According to Rule 809.1.c, the Deflect keyword is functionally short for: "Spells and abilities an opponent controls that choose me cost an amount of Power equal to [Deflect Value] more to play as an additional cost." With a total Deflect Value of 2, the additional cost to target Vex is 2 Power.

Summary

Because the Deflect values are summed per Rule 809.2, the total cost to choose Vex with an opponent's spell or ability is 2 Power. This Power can be of any Domain, as per Rule 809.1.c.1.

Note: I could not fully verify this ruling against the retrieved rules, as the specific text for 'Allay, Eager Admirer' was not present.