Sigourney Weaver enjoyed one of her last free nights last week before her Broadway show “Vanya and Sonia and Masha and Spike” begins previews tomorrow. Weaver was at the New York Botanical Garden’s annual Orchid Dinner , where she was overheard saying she’ll be working on the show from 2 p.m. to 11 p.m., but will make time for friends after hours. Designers such as Richard Mishaan created table displays for guests, including Alexandra Lebenthal, Gillian Miniter and Leesa Rowland.