Vaif is the second boss in stage 5 in Gradius III after Dogas. It is actually not one big Moai, but 6.
The 6 Big Moais spit smaller Moai just like Big Moai. However, they get bigger when hit until they explode.
Successfully killing all six heads is incredibly difficult, as not only do their attacks overwhelm and reduce maneuverability, but their mouths are positioned in ways that are not easily hit by most weapons. The best option is simply to destroy the majority of the heads to reduce their attack power and then focus on dodging the little Moais (don't shoot them) until the battle times out.