Does Phantom Lancer's illusions use aura's, like from Vladamir's? Does the lifesteal contribute to PL's health? Do they use aura armor reduction? I'm also curious about Desolation armor reduction, or do the PL illusions only do damage?
Check out http://www.dota2wiki.com/wiki/Illusions to get a understanding what illusions can do and whatnot.
Lifesteal does not work on them.
Illusions will bestow any auras that the original hero has. However, with the exception of auras that increase Attack Speed or Movement Speed, they usually will not benefit from them.