If Earth's axis had no tilt, we'd only have spring and fall for seasons, right? But which hemisphere would get the vernal equinox and which would get the autumnal?

Answer 1

The seasons are not caused by the earth being different distances from the sun: the earth's orbit is only slightly elliptical - it is essentially circular.

The seasons are caused be the axial tilt of the earth. So if there were no tilt, there would be only 1 season (or one might as well say no seasons).