The second aliya of Vayeira begins at 18:15, with the end of the Sarah-denying-having-laughed episode. It then goes on to focus on S'dom and the other cities. This happens in the middle of a column, not near a "paragraph" break of either sort, so the division isn't dictated by those boundaries so far as I can tell. Nor is 18:15 a "down" note that we wouldn't want to end an aliya on.
So why is 18:15 the first verse of the second aliya and not the last verse of the first one?