  Very massive stars (VMS) up to 200-300 $M_\odot$ have been found in the Local
Universe. If they would lose little mass they produce intermediate-mass black
holes or pair-instability supernovae (PISNe). Until now, VMS modellers have
extrapolated mass-loss vs. metallicity ($Z$) exponents from optically-thin
winds, resulting in a range of PISN thresholds that might be unrealistically
high in $Z$, as VMS develop optically-thick winds. We utilize the transition
mass-loss rate of Vink and Gr\"afener (2012) that accurately predicts mass-loss
rates of Of/WNh ("slash") stars that characterize the morphological transition
from absorption-dominated O-type spectra to emission-dominated WNh spectra. We
develop a wind efficiency framework, where optically thin winds transition to
enhanced winds, enabling us to study VMS evolution at high redshift where
individual stars cannot be resolved. We present a MESA grid covering
$Z_\odot/2$ to $Z_\odot/100$. VMS above the transition evolve towards lower
luminosity, skipping the cool supergiant phase but directly forming pure He
stars at the end of hydrogen burning. Below the transition, VMS evolve as
cooler luminous blue variables (LBVs) or yellow hypergiants (YHGs), naturally
approaching the Eddington limit. Strong winds in this YHG/LBV regime --
combined with a degeneracy in luminosity -- result in a mass-loss runaway where
a decrease in mass increases wind mass loss. Our models indicate an
order-of-magnitude lower threshold than usually assumed, at $Z_\odot/20$ due to
our mass-loss runaway. While future work on LBV mass loss could affect the PISN
threshold, our framework will be critical for establishing definitive answers
on the PISN threshold and galactic chemical evolution modelling.
