I think Vaizards' are waiting for the two sides to annihilate/weaken each other just to make an entrance and killing the survivors. When we talk about arrancars and vaizards we think they're the same: it can be true on the technical side, but they come from very different contexts.
In Hueco Mundo arrancars are accepted without problems: many hollows see hybridization as a way to increase their power, without any moral problem. In SS the situation is totally different: increasing one's own power through hollowification is seen as a mortal sin and thus the vaizards were sentenced to death. The arrancars have a place to return to, the vaizards not!
This is the reason they have no interest in helping SS: at best they can be a third hostile party, at worst they can be allied with Aizen. In Hueco Mundo they would get accepted without many problems: we already know it because Ulquiorra was planning to recruit Ichigo, if the possibility would be there.
Yeah, I kinda agree with the hostile towards both sides idea. THat's been one of the possibilities that I've been pondering. Instead of having the VIzards show up to battle Aizen's supposed VL army on the SS side, maybe they'll show up and attempt to annihilate both sides, after all, SS betrayed them, and Urahara does describe them explicitly as a "criminal organization". I don't see them allying with Aizen though.
I think they'll join Aizen by making the key. They were seen leaving their werehouse which was just transported to SS... They will help Aizen, no doubt... But to how far
At the end of the Pendulum Arc we see Urahara helped Vaizards by a great deal, yet he states he doesn't know their purposes and their location, while talking to Isshin. He even adds they're "rogue shinigami", which isn't exactly a nice compliment. I think lots of things happened after they left SS with Urahara and these events make vaizards walking their own way on their own, leaving Urahara behind.
The vizards are neither good or bad. They're an independent force. They are indebted to Urahara and would side with Urahara at any time.
Proof of this is shown at the end of the Gaiden Arc. When he says that they are in debt to Aizen as well, he meant it by destroying him.
Aizen took their lives from them. SS was their home and because of Aizen's experiments, they lost everything. Urahara gave up everything to save them. If it wasn't for him, they would've died and they would never have gotten the power they have now. Now they have another chance to live, and this time more power than they had before. Enough power to take down some very powerful enemies.
However, I do agree, they definitely have more reason to hate Aizen than like him, so I don't see them siding with Aizen in the winter war. However, they also arn't exactly fond of the soul society. Central 46 were idiots and jackasses, and the rest of SS didn't exactly stick up for them, besides Urahara, Tessai, and Yoruichi of course.
Here are what I consider the obvious possibilities for the direct involvement of VIzards in FKT, and my opinion on how possible they are:
Vizards side with Aizen:
I doubt this, as previously stated, Aizen did more to hurt them than to help them, in fact, if Yoruichi didn't save them, they would all have been executed because of Aizen.
Vizards side with SS:
This is a little bit more likely than the Vizards siding with Aizen. As Urahara/Yoruichi tend to be supporting the SS for the most part (well, post-SS arc, the SS has collaborated a lot with Urahara), same with Ichigo. So if they do side with SS, I think they would do it for Ichigo, Urahara, and Yoruichi, not for SS itself.
Vizards side with neither:
This is also a valid possibility in my book. THe vizards aren't exactly fond of either the SS or Aizen, so I wouldn't be surprised if they showed up and started fighting both sides.
This is my evidence as to why orihime was captured to big with before the war started. The visored army could not afford to be found in SS