[^_Usagi_^]

Spoiler:

Could they know who isshin real background?

I think it's safe to say that they don't know who he is. cause in one chapter, when Isshin was fighting the Grand Fisher & his reatsu sky roketed, Shinji (I think that's his name) was like "Who's reatsu is that? I don't recognise it." So they obviously don't know him.

Spoiler:

know the Vaizards in the first place

the Urahara "team" knows of Vaizards, since Urahara or Isshin said "They already contacted him, as expected "(or something like that. GAD! I really need to check the chapters for this). But I'm not sure if the Vizards know of the previous.