Actually this should be happening. If you change the magnification or position or rotation of the image in interlaced video you should get interlacing artifacts. Consider, in an unaltered frame, first lines 2, 4, 6, 8 ... will be displayed, then hafl a frame later lines 1, 3, 5, 7 will be displayed. This is fine because the even lines were recorded first then the odd lines half a frame later, and the motion is smooth. Now, if you zoom in to 200%, you'll end up with line 1 being displayed on line 2, line 2 on line 4, line 3 on line 6, line 4 on line 8, all at the same time. Then half a frame later you'll get line 1 on line 1, line 2 on line 3, line 3 on line 5, line 4 on line 7, all at the same time. So, you'll end up with All the lines being displayed during both halves of the frame, out of sync with the order they were recorded in. If you use different magnification ratios, move the picture around, or rotate it, the situation can get even worse and more bizarre.

As Spot suggests, using Reduce Interlace Flicker can help. Vegas will blend or otherwise combine the two fields of the frame before cropping or moving it. This can smooth out the image a lot in most cases, but it won't fix everything. It also reduces the effective resolution and sharpness of the image.