Can we cite from viXra?

You can, without doubt, but it might not be a good idea.

The first question is, of course, if the content of the paper is good.

The second question is, if you can find a more reliable source. Given the reputation of Vixra, almost any other source is more reliable, generally speaking.

I would suspect that, most of the time, when something is posted on viXra, either the content is not of high quality or you can find comparable material elsewhere.

But if you do happen to find a gem on viXra, certainly cite it. Just be very, very certain of the quality.

You haven't specified your field, so I don't know whether any of this applies.

If you want to cite the paper as bringing evidence for a theory you're proposing (or defending, or attacking), you should make sure that the content is good, and not blindly trust it - it's not peer-reviewed, and it's on viXra, after all.

If it contains a theoretical point that you agree with, or want to use in your dissertation, then not only can you cite it, it's your duty to cite it, just like any other source.