  The large crescents imaged by ALMA in transition disks suggest that azimuthal
dust trapping concentrates the larger grains, but centimetre-wavelengths
continuum observations are required to map the distribution of the largest
observable grains. A previous detection at ~1cm of an unresolved clump along
the outer ring of MWC758 (Clump1), and buried inside more extended sub-mm
continuum, motivates followup VLA observations. Deep multiconfiguration
integrations reveal the morphology of Clump 1 and additional cm-wave components
which we characterize via comparison with a deconvolution of recent 342GHz data
(~1mm). Clump1, which concentrates ~1/3 of the whole disk flux density at ~1cm,
is resolved as a narrow arc with a deprojected aspect ratio Chi>5.6, and with
half the azimuthal width than at 342 GHz. The spectral trends in the morphology
of Clump1 are quantitatively consistent with the Lyra-Lin prescriptions for
dust trapping in an anticyclonic vortex, provided with porous grains
(f~0.2+-0.2) in a very elongated (Chi~14+-3) and cold (T~23+-2K) vortex. The
same prescriptions constrain the turbulence parameter alpha and the gas surface
density Sigma_g through log10( alpha x Sigma_g /g/cm2)~-2.3+-0.4, thus
requiring values for Sigma_g larger than a factor of a few compared to that
reported in the literature from the CO isotopologues, if alpha <~ 1E-3. Such
physical conditions imply an appreciably optically thick continuum even at
cm-wavelengths (tau(33GHz)~0.2). A secondary and shallower peak at 342GHz is
about twice fainter relative to Clump1 at 33GHz. Clump2 appears to be less
efficient at trapping large grains.
