If those prebiotic molecules are concentrated in areas where thermal currents pass new chemicals by at high rates, where temperatures are conducive to reactions, and where mineral surfaces provide substrates for biopolymers, I would think the dilution problem is smaller than you imagine.
Easy enough to test. The chemistry involved is happening at this very moment unlike the Miller-Urey experiment. The prebiotics may be eaten by the living but they should be produced at exactly the same rates and types as the hypothetical abiotic ocean. The minerals available should not be very different now as the vents are fresh crustal material.