Somewhere on the forum some years ago someone reported trying to load VSTs in RealBasic (before it was Xojo). If I recall correctly, they were able to load a VST nominally, but the VST interface would not display, and there was nothing said about handling the audio. They gave up and used X-Code instead. A lot has changed since then, but I don't know of any more recent attempts to work with VSTs reported here.
If you are okay with Mac only (and in my opinion a steep learning curve), the MBS plugin has classes for working with AU plugins. There are no example projects and I have yet to seriously try to use them because without any examples it looks too complicated to me, but I am also interested.
EDIT: I just noticed, I misread your question :) I think definitely no, you can't *write* an AU or VST using Xojo.