My take on this is No you many not descend to the MSA. I base this answer on the following:
(US rules only)
14 CFR 91.175 - Takeoff and landing under IFR
(i) Operations on unpublished routes and use of radar in instrument approach procedures. When radar is approved at certain locations for ATC purposes, it may be used not only for surveillance and precision radar approaches, as applicable, but also may be used in conjunction with instrument approach procedures predicated on other types of radio navigational aids. Radar vectors may be authorized to provide course guidance through the segments of an approach to the final course or fix. When operating on an unpublished route or while being radar vectored, the pilot, when an approach clearance is received, shall, in addition to complying with § 91.177, maintain the last altitude assigned to that pilot until the aircraft is established on a segment of a published route or instrument approach procedure unless a different altitude is assigned by ATC. After the aircraft is so established, published altitudes apply to descent within each succeeding route or approach segment unless a different altitude is assigned by ATC. Upon reaching the final approach course or fix, the pilot may either complete the instrument approach in accordance with a procedure approved for the facility or continue a surveillance or precision radar approach to a landing.
Different countries may have different rules but in the US there was a famous accident at Washington Dulles. A TWA 727 crashed short of runway 13. It was a CFIT accident where the aircraft descended to the initial approach altitude while on vectors. There was ridge line between the aircraft's position and the runway and they hit just below the crest. This was after cockpit voice recorders but before ground proximity warning systems.