How submarines can add VLS without piercing the pressure hull

Does adding vertical launch cells dangerously increase the number of holes in a submarine's pressure hull?

Not necessarily; a large modern boat can place VLS between the outer hull and pressure hull so the cells do not pierce the main pressure boundary.

The traditional objection to VLS is that every opening complicates a submarine's pressure hull. Modern nuclear boats have grown wide enough to provide substantial space between the outer hydrodynamic casing and the internal pressure hull. Designers can place launch cells in that intervening volume so the missiles do not require a bank of full-depth penetrations through the crewed cylinder. Pressure-hull integrity therefore becomes a manageable design problem instead of a permanent reason to reject VLS.