I just finished the final episode and going back to Aqua's quest I noticed that MF was able to fight when I did not be Vanitas spirit, which is required. Has this happen to any of you?

yes, me too.

if you beat VLS on another character MF becomes open on all characters

Sounds like a glitch to me

if you beat VLS on another character MF becomes open on all characters

 

I have not because Aqua was my highest level character so I chose her for the secret bosses first

then vanitis linger was just opitionl

@post above mine: Dude, of course VLS was optional -_- You didn't have to defeat him to go onto the story mode

Anyways, sounds like a glitch to me.

Do you have multiple files? Unlocking things tends to glitch up if you have more than one of each characters files.

Actualy I dont think that its a glitch because the same thing happen on kh1 with kurt siza and phantom , 2 optional bosses at the same time its not a glitch

Considering Kurt Zisa and Phantom are unlocked under the same conditions, yes, it is a glitch.

if you beat VLS on another character MF becomes open on all characters

 

Even though I'm not for sure, I think he's correct.