The difference with the American Veep is that s/he is elected on a ticket with the pres. As you know, the Veep exercises more or less power. Cheney is very much on the more side.

In fact, there is no equivalent in the American constitution to the French PM. Or imagine that, when the president appoints the Sec of State, of Defense, the Attorney General, etc, he also appoints a coordinator of executive activity. Sounds stupid because the president is in fact (with more or less help from the Veep) that coordinator.

My personal view is that the French system on this point is full of BS. The PM is there to support a polite fiction, which is that the president is above the fray and doesn't mind the shop. The PM's work is real, but his main function is to make it look as if things are getting done, and to take the rap if not. He is often a lightning conductor for the president -- he gets fired when things go wrong, the president sails on regardless.

That leads to an answer about whether it's a vote of confidence to be appointed PM. Yes. But it's a very risky position. Ideally, a presidential candidate would want to keep away from it. (So why did Villepin take it on? No doubt because he has a high opinion of his ability to make France fall in love with him [his actual words on this seem to have been more buccaneering...])

The US VP analogy is possible but potentially confusing since for it to be more accurate you'd have to imagine the VP as representing the senate majority (actually it would have to be the congressional majority) not necessarily the president's party or choice. France had many years of "cohabitation" governments in which the president was from the right or left and the PM was from the opposite party which was the ruling majority in parliament. Presidential terms in France were for 7 years up until 2001 when they were reduced to 5 years. This was plenty of time for the electorate to decide they had had enough and change the parliamentary majority thus forcing a cohabitation government.

The brookings institute's web site has an article by Olivier Duhamel, Professor of Constitutional Law and Political Science at Sciences Po Paris, that discusses the French electoral system and provides this graph of the 5th republic presidents and prime ministers illustrating the cohabitation of red and blue: