Williams, fresh off her fifth Wimbledon title last week, stayed hot at Stanford's Taube Family Tennis Stadium Saturday, notching a quick 6-1, 6-2 win over ninth-seeded Romanian Sorana Cirstea.
Williams, the defending Bank of the West champion, faced just one break point in the one-sided affair, which sent her into Sunday's title match.
There she'll face lucky loser Vandeweghe, who recorded a 6-2, 3-6, 6-2 triumph over No. 5 seed Yanina Wickmayer of Belgium despite five double-faults.
The 20-year-old American became the first lucky loser to appear in a WTA final since Melinda Czink reached the Canberra final in 2007.
It will be the first final for the niece of former NBA player Kiki Vandeweghe.