If Brown wins, what is the status of Kirk's vote on the healthcare bill?

[J. J.]

If Brown wins, the Democrats would consider having the house approve the Senate bill as it is, thus sending it to the President. However, would that be legal? Would Kirk's vote be as if it didn't exist?

Yes, Kirk was a legitimate Senator at the time the vote was cast.  It obviously did exist, and the chair declared it adopted.  

That was only on cloture where his vote made a difference.