Merck Adopts Key Managers Benefits Plan  NEW YORK (Reuters) - Merck   Co. Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=MRK.N target=/stocks/quickinfo/fullquote"&gt;MRK.N&lt;/A&gt;, whose shares  have tumbled since the withdrawal of its blockbuster arthritis  drug Vioxx, said on Monday it has adopted a severance benefits  plan aimed at protecting key managers in the event the company  is taken over or liquidated.