Vastly rhymes with:
astley, ghastly, lastly, astley, costley, costly, dishonestly, earnestly, firstly, ghastly, ghostly, honestly, justly, lastly, manifestly, modestly, mostly, priestley, priestly, robustly, steadfastly, unjustly, westley
Vastly sounds like:
vastola, vegetal, vestal, vogtle