We have observed a magnetic vortex lattice (VL) in BaFe2(As_{0.67}P_{0.33})2
(BFAP) single crystals by small-angle neutron scattering (SANS). With the field
along the c-axis, a nearly isotropic hexagonal VL was formed in the field range
from 1 to 16 T, which is a record for this technique in the pnictides, and no
symmetry changes in the VL were observed. The temperature-dependence of the VL
signal was measured and confirms the presence of (non d-wave) nodes in the
superconducting gap structure for measurements at 5 T and below. The nodal
effects were suppressed at high fields. At low fields, a VL reorientation
transition was observed between 1 T and 3 T, with the VL orientation changing
by 45{\deg}. Below 1 T, the VL structure was strongly affected by pinning and
the diffraction pattern had a fourfold symmetry. We suggest that this (and
possibly also the VL reorientation) is due to pinning to defects aligned with
the crystal structure, rather than being intrinsic.Comment: 9 pages, 9 figure