  In this article we describe simulations of ZZ, WZ and WW production based on
the positive weight next-to-leading-order matching scheme, Powheg, in the
Herwig++ event generator. Building on earlier efforts within the Herwig++
framework, the simulation includes a full description of truncated showering
effects, required to correctly model soft, wide angle, emissions in
angular-ordered parton showers. We utilize simple relations among each of the
diboson cross sections, holding to order alpha_S, in constructing the
simulation. Spin correlation effects are also included in the decays of the
vector bosons at the tree order. A large part of this work is concerned with a
full and thorough validation of the simulations through comparisons with
alternative methods and calculations.
