OH maser emission from the supergiant star VXSgr has been mapped in both senses of circular polarization at 1612MHz with an angular resolution of 0.14arcsec and a velocity resolution of 0.36kms^-1 using the MERLIN array. The maser emission arises in an envelope of inner radius ~1000au and outer radius ~1900au expanding at velocities of 20 and 22kms^-1 respectively. The envelope structure is complex, but most of the 1612-MHz data can be accounted for by an axisymmetric radial outflow in which the masers are concentrated toward a thick circumstellar disc. One Zeeman pair was found that implies a magnetic field strength of -1.1mG at the location of the 1612-MHz masers. Several maser features exhibiting complete circular polarization have been detected. The maser features with circular polarization higher than 50 per cent are preferentially located within the north-east and south-west quadrants of the envelope. The non-random distribution of highly polarized features suggests a dipole magnetic field configuration. The projection of the magnetic axis in the plane of the sky is 30+/-20 deg, which is quite similar to that of the polar axis of the circumstellar disc. Such a configuration suggests a bipolar flow in which mass loss may be magnetically channelled.