Assuming that XtA is 25 and Vanitas is 15, and that AtW would find him 10 years later and makes him an apprentice, Vanitas can be XtA.
Except Ansem finds Xehanort right after BBS.
I mean, how can Ienzo (look at his picture he's a kid!!) suddenly has an adult nobody?
Zexion's (and possibly Ienzo's) powers dealt with illusions. Zexion could simply be making himself look older or Ienzo making himself look younger.
Lea, Isa, and Braig are younger than how their nobodies look.
Actually, Braig only looks younger because he lacks the eye patch, scar, and gray hair.
As for Isa and Lea, they more than likely became Nobodies a decent amount of time after the original six.
So it's safe to say that Vanitas is XtA
It most certainly is not.
A: Xehanort has far too much in common with Terra and MX.
B: Vanitas has next to nothing connecting him to Xehanort.
C: Vanitas actually has a lot of things preventing him from being Xehanort.
he was found 10 yrs after BbS.
Right after BBS.