The design is the features of configuration, shape, ornamentation and pattern of the portion of the VACANCY SENSOR shown in solid lines in the drawings. The portions shown in stippled lines do not form part of the design.Figure 1 is a perspective view of the vacancy sensor;Figure 2 is a front view of the vacancy sensor shown in Figure 1;Figure 3 is a left side view of the vacancy sensor shown in Figure 1;Figure 4 is a right side view of the vacancy sensor shown in Figure 1;Figure 5 is a top view of the vacancy sensor shown in Figure 1;Figure 6 is a bottom view of the vacancy sensor shown in Figure 1;Figure 7 is a perspective view of a first variant of the vacancy sensor;Figure 8 is a front view of the vacancy sensor shown in Figure 7, the left side, and the rightSide, top, and bottom views, respectively, of the first variant being identical to the leftSide, right side, top, and bottom views of the vacancy sensor shown in Figure 1;Figure 9 is a perspective view of a second variant of the vacancy sensor;Figure 10 is a front view of the vacancy sensor shown in Figure 9, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 1;Figure 11 is a perspective view of a third variant of the vacancy sensor;Figure 12 is a front view of the vacancy sensor shown in Figure 11, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 1;Figure 13 is a perspective view of a fourth variant of the vacancy sensor;Figure 14 is a front view of the vacancy sensor shown in Figure 13;Figure 15 is a top view of the vacancy sensor shown in Figure 13;Figure 16 is a bottom view of the vacancy sensor shown in Figure 13, the left side andright side views, respectively, of the fourth variant of a vacancy sensor being identical tothe left and right side views of the vacancy sensor shown in Figure 1;Figure 17 is a perspective view of a fifth variant of the vacancy sensor;Figure 18 is a front view of the vacancy sensor shown in Figure 17, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 13;Figure 19 is a perspective view of a sixth variant of the vacancy sensor;Figure 20 is a front view of the vacancy sensor shown in Figure 19, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 13;Figure 21 is a perspective view of a seventh variant of the vacancy sensor;Figure 22 is a front view of the vacancy sensor shown in Figure 21, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 13;Figure 23 is a perspective view of an eighth variant of the vacancy sensor;Figure 24 is a front view of the vacancy sensor shown in Figure 23;Figure 25 is a left side view of the vacancy sensor shown in Figure 23;Figure 26 is a right side view of the vacancy sensor shown in Figure 23;Figure 27 is a top view of the vacancy sensor shown in Figure 23;Figure 28 is a bottom view of the vacancy sensor shown in Figure 23;Figure 29 is a perspective view of a ninth variant of the vacancy sensor;Figure 30 is a front view of the vacancy sensor shown in Figure 29, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 23;Figure 31 is a perspective view of a tenth variant of the vacancy sensor;Figure 32 is a front view of the vacancy sensor shown in Figure 31, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 23;Figure 33 is a perspective view of an eleventh variant of the vacancy sensor;Figure 34 is a front view of the vacancy sensor shown in Figure 33, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 23;Figure 35 is a perspective view of a twelfth variant of the vacancy sensor;Figure 36 is a front view of the vacancy sensor shown in Figure 35;Figure 37 is a top view of the vacancy sensor shown in Figure 35;Figure 38 is a bottom view thereof, the left and right side views, respectively, of thetwelfth variant of the vacancy sensor being identical to the left side and right side viewsof the vacancy sensor shown in Figure 23;Figure 39 is a perspective view of a thirteenth variant of the vacancy sensor;Figure 40 is a front view of the vacancy sensor shown in Figure 39, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 35;Figure 41 is a perspective view of a fourteenth variant of the vacancy sensor;Figure 42 is a front view of the vacancy sensor shown in Figure 41, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 35;Figure 43 is a perspective view of a fifteenth variant of the vacancy sensor;Figure 44 is a front view of the vacancy sensor shown in Figure 43, the left side, the rightside, top, and bottom views, respectively, of the first variant being identical to the leftside, right side, top, and bottom views of the vacancy sensor shown in Figure 35;Drawings of the design are included.