Boyle’s law is applied for (almost) vacuum or not

Seems like a strange question, I want to know if Boyle’s law is applied for (almost)vacuum or not. Imagine you have a very very small sac which is filled completely with a fluid, and there is no air in it. This sac is elastic so you can expand it, you put this sac in the air and you want to find its pressure when you expand it.First it is said that the pressure is below than atmosphere and this pressure drop occurs because the sac is filled with fluid, which cannot expand to fill the slightly larger volume. Therefore, a vacuum exists in the infinitesimal space in the slightly expanded sac which is not occupied by fluid, producing a small drop in pressure below atmospheric pressure(It was written in a book but I can’t understand it very well).My question:if now I expand this sac more, if its pressure drops more is that “because of Boyle’s law”? If it is not because of Boyle’s law so how can I justify the more drop in pressure? Thank you for your attention.