Venetia Digby, also called Lady Frances Cranfield, Countess of Dorset
The Lewis Walpole Library at Yale Univeristy that has this as Lady Frances Cranfield, Countess of Dorset by John Hoskins after van Dyck. The caption says Venetia Digby. Venetia Digby's article in thepeerage.com is here. The article in thepeerage.com has a deathbed portrait of a similar looking Lady named Venetia Digby who died of mysterious causes in 1633. Her husband was devastated by her death according to her Wikipedia article.