should psionic communication be considered a use of witchcraft? Lots of non-witches use it.
This is one of those things that has become muddied as the Vlad stories progressed. It is stated early on that witchcraft is harnessing of psionic abilities... Although it is never said that this is the ONLY way to harness psionic powers.
I would say that this still qualifies as witchcraft... When used by a witch. Certainly it should not be stated that psionic communication is ALWAYS witchcraft, but someone like The Warlock would not have the ability to communicate psionically without being a witch, so it should still count as an exercise of his abilities as a witch.
It is also said in the Khaavren romances that the Orb allows telepathy that is not psionic in origin. Probably most of the non-witch uses of telepathy use this method, rather than psionic contact. When Vlad refers to this type of communication as "psionic" it's probably just because that's the method he's used to. (I don't remember Paarfi ever uttering the phrase "Psionic", tho I could be wrong.)
It's possible (maybe mentioned somewhere too) that psionic contact between witches is qualitatively different from that between sorcerors. Possibly the first draws upon their knowledge of each other and inherent psionic capabilities, the latter draws only upon the Orb and having some good idea of who the other person is.