Dream Zanarkand is an Aeon. It is an actual place, located far out at sea. Hecht went out to sea, and eventually wound up on Spira.
There was no "temporal vortex", nothing mystical. He just sailed out to sea and wound up in a new location.
Sin, probably while Jecht still hand some control, carried Auron there (Auron was dead anyway, so Sin wouldn't exactly do anything to him). The same way, just moving across the water.