At the time of this writing there is no explanation on wiktionary, and searching the web does not yield an answer, only more instances of its use:
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A random entry at the Etymological Dictionary of the Scottish Language:
To KELE, v. a. To kill.
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Random entry at latinlexicon.org:
verbero, āvi, ātum, 1 […], v. a. verber, to lash, scourge, whip …
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Examples section in Speijer's Sanskrit Syntax, 1886:
Examples: Panc. 2 द्वादशभिर्वर्षौर्व्याकरणं श्रूयते (v. a. grammar requires twelve years to be mastered), …
My intuition is that this is an outdated Latin abbreviation similar to "e.g." and "i.e.". (Perhaps something like verbis aliis?)
Edit: More examples from the same book (Speijer, Sanskrit Syntax, 1886), pages 2, 32, and 43:
शान्तं पापम् (v. a. malum absit)
उषिताः स्मो ह वसतिम् (v. a. we have passed the night)
कस्तेन सह तव स्नेह (v. a. how are you his friend?)
The English gloss is more or less a translation of the Sanskrit. The only thing distinguishing these translations from others (without "v. a.") is that they are a tad less literal.
I am convinced that this is not a problem of Sanskrit or grammar but of academic language of the late 19th century. The text is riddled with Latin abbreviations that would have been instantly apparent to the contemporary scholar.
Best Answer
OED has under A:
So v.a. stands for verb active.