Here's another link to more recent comments by Fred that gives you the quick answer - he did vote to convict on the obstruction charge (and we only needed one) but didn't feel the perjury charge fit the Founders definition of impeachable:
On Bill Clinton's impeachment, Hannity asked: Why did you vote against conviction? From Thompson: I split my vote according to what I believed to be my duty. I voted to convict on obstruction of justice, and not to convict on charges of perjury. I went back to the Founder's intent and believe they did not consider this an impeachable offense.
Ah, thanks. I can’t believe I missed the Hannity interview.