The (1, 0) band of the B4Π-X4Σ- transition of VO has been analyzed rotationally, from Doppler-limited discharge emission and laser excitation spectra. The B4Π, v = 1 level is heavily perturbed by a vibrational level of the a2Σ+ state and, with the extra complexity caused by the 51V nuclear hyperfine structure, could be analyzed fully only by extensive wavelength-resolved laser-induced fluorescence experiments. Making use of data from the corresponding perturbations in B4Π, v = 0 and assuming that the vibrational dependence of the spin-orbit matrix element can be represented by a vibrational overlap integral, it has been possible to deduce the vibrational numbering of the perturbing a2Σ+ state without having to use isotopic data. It is found that the a2Σ+, v = 0 level lies near 10412 cm-1, and that B4Π, v = 1 is perturbed by a2Σ+, v = 3. The a2Σ+ state comes from the same electron configuration, (4sσ) (3dδ)2, as the X4Σ- ground state. The phases of the spin-orbit matrix elements between 4Π and 2Σ+ states must be chosen consistently in order to obtain a correct understanding of the rotational perturbations. A consistent set of phases is obtained with 〈2Σ+, F2 | HSO | 4Π1/2, f〉 = 〈2Σ+, F1 | HSO | 4Π12, e〈 = [formula]〉2Σ+, F2 | HSO | 4Π- 1 2, f〈 = -〉2Σ+, F1 | HSO | 4Π- 1 2, e〈 = A/2, where A = 〉2Σ+ П Hspin-orbit П 4Π〈.