Article from Radio Times announcing Maureen O’Brien coming to Doctor Who, December, 1964.
Vicki is my favorite TARDIS crewmember ever, because she was the first in the modern mold: she was lively, energetic, bubbly, and had way better chemistry with the rest of the cast than the actress she replaced. That’s not a slam, chemistry is unpredictable - and people just light up when Vicki entered the scene, just watch the actors’ faces. It’s no exaggeration to say Vicki’s effervescence shifted the tone of the show (Doctor Who episodes before her were vaguely ponderous and Outer-Limits-ish).
With all due respect to the original three TARDIS crewmembers, who were all great, their group dynamic was a false start that was never really duplicated. In the long term of the show, Vicki was the one from that era that mattered. To use an evolutionary metaphor, Vicki was the “common ancestor” of the modern companion: she had pluck Susan didn’t have (Susan was never reliable in a crisis), took initiative, suggested ideas, and even took charge at times. You can see a straight line of descent from Vicki all the way to TARDIS crew members like Sarah Jane Smith and today’s sidekicks.