I like Bujold as well, but in regards to Mary, Vanessa was perfectly cast. She had a tremulous quality that suited the character wonderfully, vs. Bujold overt strength and boldness. Also, while Bujold had a pert prettiness, Vanessa was exquisite, and physically suited Mary perfectly, who was also blond and renowned as one of the most beautiful princesses or queens in Europe.
Cannot fathom Farrow as Elizabeth. They may as well have cast Marlo Thomas.

She did very well, in 'Coma'.
Carpe Noctem!

That's correct. But Genevieve Bujold was afraid of being type cast as a British queen and refused the rolewhich led to her being sued by Universal Pictures and probably derailed her very promising career. By the time she returned for a "comeback" and did EARTHQUAKE to please Universal, the damage was done. As much as I'd have loved to have seen Bujold as either Mary or Elizabeth, it was not to beand Redgrave was outstanding as the Queen of Scots.