  A magnon Bose-Einstein condensate in superfluid $^3$He is a fine instrument
for studying the surrounding macroscopic quantum system. At zero temperature,
the BEC is subject to a few, distinct forms of decay into other collective
excitations, owing to momentum and energy conservation in a quantum vacuum. We
study the vortex-Higgs mechanism: the vortices relax the requirement for
momentum conservation, allowing the optical magnons of the BEC to transform
into light Higgs quasiparticles. This observation expands the spectrum of
possible interactions between magnetic quasiparticles in $^3$He-B, opens
pathways for hunting down elusive phenomena such as the Kelvin wave cascade or
bound Majorana fermions, and lays groundwork for building magnon-based quantum
devices.
