It seemed to her, as she did so, that the tension of Mrs. Vandemeyer's attitude relaxed.
As she did so, it appeared the tension of Mrs. Vandemeyer's attitude relaxed. 
mnli hypothesis: As she did so, it appeared the tension of Mrs. Vandemeyer's attitude relaxed.  premise: It seemed to her, as she did so, that the tension of Mrs. Vandemeyer's attitude relaxed.