import yt

data_ds = yt.load(<original snapshot>)
halos_ds = yt.load("catalog/catalog.0.h5")
for i in range(halos_ds.r['particle_identifier'].size):
    center = data_ds.arr(halos_ds.r['particle_position'][i].to('unitary'), 'unitary')
    radius = data_ds.quan(halos_ds.r['virial_radius'][i].to('unitary'), 'unitary')
    sphere = data_ds.sphere(center, radius)
    pos = sphere['particle_position']