Which model of Mauser-Vergueiro is it, a 1904 or a 1904/39? The Trigger Guard screws on the M1904/39 have capture screws as well which can be tight on there (one trick can be moving the main screw slightly to take the pressure off the capture screw which then allows you to remove them). Oil it if you need to as that will help.

Pictures help with this type of thing. Here is a photo of that type of bolts both assembled and disassembled.