It goes like this.
In the end everything works out. Vanitas is banished, Xehanort is flung into the dark pit he deserves, and Aqua, Terra, and Ventus meet on the field of battle with nary a wound nor qualm between them.
They return home to find a joyful greeting waiting for them. "You have all done so well," the Master says. "I am proud of all three of you."
That night they all watch the sky from their own little world and know that each star twinkling out in the sky is safe and whole. They always will be.
Years go by and they grow older. One fine, golden day the Master passes his mantle to Aqua; Terra kisses her hand at the ceremony and Ventus claps so hard his hands go numb. Eraqus spends the rest of his days in peaceful retirement and drinks tea with Yen Sid every other Thursday.
All is well, and no major darkness ever threatens them again. Ventus divides his time between home and visiting his friends off world, delighting especially in trips to Disney Town. His most pressing trouble is an occasional headache – Aqua just says it's a side effect, of what she won't say, and to not worry about it. He doesn't. Ventus is happy, safe, and has everything he ever wanted.
He is never lonely, never at all, as he sits day-by-day dreaming his wishes into oblivion.