As pointed out by another answer, what matters is the field strength in volts per metre. The energy would have to be imparted within the incredibly short lifetime and travelling distance of the virtual pair.
Bringing the power lines closer together increases the field strength. But many orders of magnitude before any particle-pair interactions, the field will begin to ionise the air around the lines. Initially this has two main consequences; current begins to leak across the gap and at night the air around the negative line glows gently as electrons from the line recombine with the air ions. The effect looks very pretty. Bring them closer still and you eventually get an avalanche discharge, an electric arc, between them.
Even in a vacuum they will arc across, the voltage collapse and the lines break or coalesce long, long before any particle pairs can be created.