Edit: If I understand you correctly now, the motivation for vacuousness is not your main issue with it, but by the time you wrote your clarification comment I had already written up this answer, perhaps it is of interest anyway:
The motivation for why counterfactuals are permitted to be vacuously true is given in the original Lewis (1973) "...
Lewis invites us to consider some counterfactual conditionals:
Φ1 □→ Ψ
Φ1 ∧ Φ2 □→ ¬Ψ
Φ1 ∧ Φ2 ∧ Φ3 □→ Ψ
He points out that in the way such conditionals are ordinarily used, all of these may consistently be true, which is one way of saying that counterfactual conditionals are typically non-monotonic. This creates difficulties for the position that ...