Meh, I'm not sure how much this actually means given how much Xehanort's gone back and contradicted himself... I mean, the "right" way now is to smash 7 lights and 13 darknesses together, even if the components of said light and darkness aren't totally pure themselves. I think what he was trying to go for was light and darkness of equal power. Splitting the same heart into pure light and pure darkness might've made it easier to get the powers to be equal.
I'd like to throw into the puddle that Xehanort may not necessarily "contradicting" himself when we take into account that Xehanort throughout the entire series has shown that he likes to
just experiment around.
He creates a certain starting scenario and then leans back and watches how it turns out.
If it goes well and is a success: "Yay, goals reached, Kingdom Hearts here I come."
If it fails: "Lol, too bad for the pawns involved, lets look into the next possible variant that may work."
So when he speaks about the "pure light" and "pure dark" in the BBS Reports, he's talking so confidently about it because at current time and at the time when he wrote that particular report, he's convinced that this may be the variant with the highest chance to succeed apart from the 7vs13-method which during that time he deems too complicated and time-intensive to try.
Apart from that though, if this variant would become impossible to truly execute, I wouldn't put it beyond Xehanort to say "meh, let's see perhaps a 99% light heart might result in something workable as well."
Of course it is also possible that Vanitas was acting on his own volition here and it was
him who forgot that the "purity"-part has to be absolute.
Maybe when meeting Xehanort again offscreen the old coot chews Vanitas out not only for his stupid actions trying to end Ven, but also for considering Aqua a viable alternative since while it might result in
something, the chances of it being successful would certainly be lower.
Instead, Xehanort orders Vanitas to intensify the "training" for Ventus and to end Aqua the next time they meet.
Indeed, when Vanitas and Aqua meet again he does remark about Ventus' progress and claims
he decided that she's no more needed as a backup (Vanitas' own ego/pride and his disdain for Xehanort would certainly prevent him from admitting to a third party that he's carrying out the old coot's orders).
Also, while Nomura interviews certainly are in
many cases good for getting explanations and filling gaps in some mysteries, one should keep in mind that they are not absolute and that if there is a contradiction between an interview and actual in-game information/confirmation, the content of the game takes precedence.
I'm honestly not sure at all myself, but I think Vanitas was made because Ventus at the time wasn't strong enough and wouldn't give into the darkness. And Aqua (probably) has lots of light in her heart, and she's a strong Keyblade wielder, so I guess her being a backup would make sense?
That's more or less canon.
No matter what Xehanort tried Ventus won't give in/succumb to Darkness, but Xehanort does
need an "avatar" for the dark half of the blade in order to pursue his ambitions through his current plan/experiment.
We don't know how long Ventus was Xehanort's student before the Vanitas' creation flashbacks, but in the reports Xehanort not only remarks that Ventus is in general "too frail" to be used as a vessel but also that he's "too benign for his own good", which may among other things indicate that the Neo Shadow-attack orchestrated by Xehanort was certainly not the first attempt by the old coot to try and have Ventus give in to his Darkness.
The Neo Shadow-attack was just the last attempt in a possible row of incidents with Xehanort creating a "do-or-die"-situation because he began to lose his patience.
Throughout the whole scene Xehanort is seen to goad Ven on to give in to Darkness, just like Ansem SoD does constantly to Riku throughout the series.
When Ventus still doesn't use his Darkness or falls to it even in such a dire situation however, Xehanort outright states his disbelief/disappointment over the fact:
Master Xehanort said:
Really? You would rather die than use the power? Feckless neophyte.
You can practically hear his disdain from this assessment, his frustration that no matter what he tried, this damn kid
just wouldn't give in.
For someone more inclined towards the light Ventus' resilience/refusal to give in would probably be seen as a good/admirable trait (Eraqus, Yen Sid and probably Mickey would be proud), but since Xehanort
favors Darkness and sees it as the way to "true" power he interprets it as the failure of "a feckless neophyte"
.
When I played BBS for the first time on the PSP back in 2010 and got to this scene/statement Xehanort's reaction somehow reminded me of the reaction of Emperor Palpatine in Star Wars: Episode VI - Return of Jedi when Luke Skywalker tosses his Lightsaber aside and declares he won't get over to the Dark Side.
Both Palpatine and Xehanort are
frustrated as heck in these moments. They're both used to getting their way and now someone who's a damn insignificant weakling in their eyes successfully resist their attempts in one way or another.