It seemed to her, as she did so, that the tension of Mrs. Vandemeyer's attitude relaxed.
After she offered tea and made small talk, it appeared the tension of Mrs. Vandemeyer's attitude relaxed. 
mnli hypothesis: After she offered tea and made small talk, 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.