The perceptual evaluation of spatial audio algorithms is an important step in the development of immersive audio applications, as it ensures that synthesized sound fields meet quality standards in terms of listening experience, spatial perception and auditory realism. To support these evaluations, virtual reality can offer a powerful platform by providing immersive and interactive testing environments. In this paper, we present VR-PTOLEMAIC, a virtual reality evaluation system designed for assessing spatial audio algorithms. The system implements the MUSHRA (MUlti-Stimulus test with Hidden Reference and Anchor) evaluation methodology into a virtual environment. In particular, users can position themselves in each of the 25 simulated listening positions of a virtually recreated seminar room and evaluate simulated acoustic responses with respect to the actually recorded second-order ambisonic room impulse responses, all convolved with various source signals. We evaluated the usability of the proposed framework through an extensive testing campaign in which assessors were asked to compare the reconstruction capabilities of various sound field reconstruction algorithms. Results show that the VR platform effectively supports the assessment of spatial audio algorithms, with generally positive feedback on user experience and immersivity.