Yep it'll continue to drop like this in absolute pressure (psi):

Top: 15 , bottom: 12+15=27
Top: 10 , bottom: 12+10=22
Top: 5 , bottom: 12+5=17
Top: 0 , bottom: 12+0=12

It's impossible to go below zero at the top, so the bottom will never approach zero.

That assumes you can actually achieve such low pressure without the water being sucked out as vapour.