Viacom Posts Profit; Sets \$8 Bln Buyback  NEW YORK (Reuters) - Viacom Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=VIAb.N target=/stocks/quickinfo/fullquote"&gt;VIAb.N&lt;/A&gt;&lt;VIA.N&gt; on  Thursday said its quarterly profit from continuing operations  rose 12 percent on higher cable network advertising sales and  announced a larger-than-expected stock repurchase, sending its  shares up 3 percent.