Why not just kill Van Damm
He's selling American secrets to the soviets... Why doesn't the professor just have him killed by a secret agent?
shareAccording to the Professor, she would be "valuable on the other side", possibly meaning that she'd be staying with Vandamm on his travels to his destination (hinted to be Soviet Russia), and she could leak out information regarding his status there and possibly his organization.
That, and it's to drive Thornhill to rise up to hero status, sneak in the villain's home, and save the girl from peril.