I'm sure "vacco" should be "vaco" as you suggest. But the meaning is more "I am free to attend to..." rather than literally "I am vacant". "Scilicet" is commonly "to wit", "that is", "namely", etc. instead of "certainly". Then I think we can put both "ad talia" and the dative gerund with "vaco". The beginning is still weird but I'm quite sure "quia" begins the relative clause governed by "redargui". I'm going to interpret the "possum ergo sane" bit as something like, "Yes, so I could---indeed, actually I already DID refute that..."

So, maybe something like:

I am capable--indeed, I proved untrue that I have time for such matters, that is, for writing childish or useless things.