From what I understand Vistaprint accepts a PDF with the design (one card), they will then handle impositioning.
From their homepage the specifications are:
Full Bleed Size (starting document size)
Finished artwork should use the full bleed dimensions for best results
3.62" x 2.12"
92 x 54 mm
1085 x 635 pixels
Document Trim Size (final size after being cut)
3.50" x 2.00"
89 x 51 mm
1050 x 600 pixels
Resolution
300 DPI
I think that there somewhere also is information about safe distance to the edge (cutting tolerances), I've looked it up before.
So, in Scribus, set up a page size of 89x51 mm, bleed 1,5 mm on all sides.