I run into the flickering all the time with vegas 6, slowing 50i to 50% and rendering to 25p. The only way I could get rid of it was to alternate frames of deinterlaced video, so I had rendered the clip with interpolate deinterlace once as lower first, once as upper first then alternated the frames by masking one track with solid white every other frame. Only took about 5 mins to set up cos you can cut and paste blocks of the single white frames for the mask.