How can I conceal a gap between a wall and newly installed vinyl flooring that I cut too short?
You can use quarter round to extend your baseboards and cover small gaps.


(source: houserepairtalk.com)  

If the gap is really large you can cut a new larger piece of the flooring and line it up with the current floor in order to create a hopefully invisible seam.  I think it generally looks better to have two large pieces with a seam instead of one large piece and one small piece.