Measurements are reported of the rate nu at which negative ions nucleate quantized vortices in isotopically pure superfluid He-4 for electric fields E, temperatures T and pressures P within the range 103 < E < 106 V m^-1, 75 < T < 500 mK, 12 < P < 23 bar (= 2.3 MPa). The form of nu(E,T) differs in unexpected ways from that observed in earlier work at higher P, exhibiting: a pronounced dip in nu(T) at ca. 0.3 K whose depth and precise position depends on E and P; an exponential increase in nu(T) at higher T, with an activation energy considerably smaller than the roton energy gap; and distinct structure in nu(E). The experimental data are discussed and analysed in terms of the macroscopic quantum tunnelling model proposed by Muirhead et al. (Phil. Trans. R. Soc. Lond. A 311, 433 (1984)). The relatively small barrier heights of ca. 2-3 K deduced from the data on this basis are construed as confirmation that the initial vortex is a loop rather than an encircling ring. The temperature dependence of nu at low pressures is interpreted in terms of a phonon-driven vortex nucleation mechanism, and values for its cross section are extracted from the data. The minima in nu(T) are ascribed to phonon damping of the tunnelling process, and the kinks observed in some of the low-temperature nu(E) curves are attributed to tunnelling of the system into the first excited state of the nascent vortex loop.