The second-largest vendor at the Westchester County-owned Playland Amusement Park could lose its contract if it continues to sell realistic-looking toy guns at a park gift shop, county officials said yesterday. Peter and Fredericka Trahanas, who own the second highest number of concession contracts at the historic amusement park, were ordered to pull several models of toy guns off the shelves Friday after County Executive Andrew Spano learned from a WCBS-TV news reporter that the plastic guns were on sale — in violation of a 2000 law pushed by his administration.