Vanitas Lingering Sentiment can be refought by talking to the sphere of emptiness again. Some might call it an "Absent Silhouette" since it looks just like the portals in Kingdom Hearts II Final Mix.
Mysterious Figure can be refought by "leaving the area" upon defeating him. By going back into the Land of Departure, you trigger the cutscene again and from then on have to win against him again but to be honest MF is there just for bragging rights and a challenging boss fight. To me he is without a doubt one of the most difficult extra bosses in the Kingdom Hearts series; And this comes from a guy who has beaten the Lingering Will in KH2FM - He was one hell of a hard boss, but he is able to be dealt with easier once you know his patterns. Oh and for the record, I have defeated all the bonus bosses in all the games so far.... MF is by far the hardest of them all... second to Julius from KH3D [Dream Drop Distance].
Finally beat VLS and MF with Ven. 1 Thunder Surge, 3 Fire Surges and 4 Curagas. Not sure if having 4 Curagas was a good idea, I think 3 would be enough, but I did it at Level 48 with Void Gear.
Just a suggestion, Curagas can easily be traded for cures/curas since I assume you have second chance/that other skill that's similar(I can't remember the name). MF is so strong that it doesn't matter how much HP the spell heals you will still be reduced to one HP. Having cures/curas vs. Curagas also means shorter command reload time and more slots available.
I already knew about that before, but... I don't know, I feel safer when I have more HP, even though it's totally pointless. And I never had a situation with Ven where I needed to heal and the Commands were on reload time, so I went with the Curagas anyway.
But with Terra... I'll need EVERY help I can get, so you bet I'll be using Cures.
Call me Mega, Megaman, X, or any variation/combination of the previously stated names. Official Liu Kang of the PS3 Mortal Kombat board.