I am not from Louisiana, but have a passing knowledge of its election law and local politics. It would be better for someone from there to greatly supplement what I am writing here.
If more details of Vitter's activities arise, and particularly if he is caught lying, it is possible (although unlikely with the tenuous Democratic majority) that Vitter might resign. (An additional reason for this being unlikely it that if other GOP officials are outed, it would be a bad precedent for them). If he does however, it would be at least a short term Democratic pickup possibility, and maybe even ultimately help the Dems add or keep two Senate seats.
More details and thoughts below....
Louisiana for the time being has a Democratic Governor Blanco. She is not running for reelection; it is assumed that current GOP Rep Bobby Jindal (who lost to Blanco four years ago) will be elected to replace her later this year.
If a Senate vacancy occurs, the Governor makes an interim appointment of anyone she chooses who is eligible. She would somewhere between likely and certainly name a Democrat.
The first round in Louisiana's state-wide elections is this October. Candidates run in an open primary for offices - if no one gets a majority, the top two compete in a runoff in November.
I am guessing - someone with knowledge of Louisana law can help here - that if a resignation took place before a certain point, the election for the remainder of the term would take place either in October of November. Would she have the option of delaying until November 08 (likely when the next scheduled elections are - again, House and Senate races are open elections with runoffs if no one runs. So an interim appointment could be very short term.
Might Vitter if he is thinking of resigning holdoff until after the next governor is sworn in?
As to Blanco's choices, who among local Democrats would be a choice that might have a chance of winning? Bring back John Breux?
One other piece of speculation - Karl Rove is trying to recruit State Treasurer John Kennedy - a Democrat - to switch parties and run against vulnerable Mary Landrieu (Democratic Senator) next year. Could Blanco be really really cute and see Rove and raise him one - appoint Kennedy to the seat with his commitment to caucus with the Democrats. thus possibly helping insure two seats (I understand that Kennedy is quite popular, and that if he is not the GOP candidate against Landrieu, she could be much less vulnarable.)
For those in Louisiana - how right wing is Kennedy? Has he supported state wide and national Democratic candidates?
Are there any other Democrats she could name apart from Breaux or Kennedy who might win?
In any event, nice to be discussing a vacancy when it doesn't involve someone dying and seriously ill for once.