Pentax X90 has a
12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor and features Prime processor. On the other hand, Pentax VS20 has a
16.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor .
Below you can see the X90 and VS20 sensor size comparison.
Pentax X90 and Pentax VS20 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Pentax X90 has 32% larger pixel area compared to Pentax VS20, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.