Complex sentence: Blue Vesta was a 150 kg Infra-Red guided weapon for attack from a pursuit angle-in the event it was largely displaced in the designs by the Mark 4 version of Blue Jay (which would enter service as Red Top).
Split into simpler sentences: Blue Vesta was a 150 kg infra-red guided weapon for attack from a pursuit angle. The Blue Vesta was largely displaced by the Blue Jay Mark 4 (which would enter service as the Red Top).