A couple of ways:
Take a snapshot and then use the "deinterlace" option in your photo editor.
Change your project properties to Progressive before taking a snapshot. When using a script, these changes are made (and then reset when finished):
Vegas.Project.Preview.RenderQuality = Best;
Vegas.Project.Preview.FullSize = true;
Vegas.Project.Video.FieldOrder = ProgressiveScan;
Vegas.Project.Video.DeinterlaceMethod = InterpolateFields;
Since you're on "Movie Studio" and can't use scripts, you can manually make these same changes before taking the snapshot and then change them back when done. The first two are important and are done by changing the preview mode to "Best (Full)". The other two are under File - Properties.
Edward Troxel