You've gone completely off the rails here and are in the historical-linguistics ditch. Etruscan words are very rare, especially by the time of Varro, and would make more sense further north. And Phoenician?

vorsum is just a pronunciation variant of versum, the Latin accusative singular masculine/neuter singular perfect passive participle of vertō, vertere, vertī, versum "to turn, pivot". In Italic languages there was a dual pronunciation of the vowel in the stem oscillating between "e" and "o," especially in the perfect system. So, in Latin, from vertere "to turn" comes the perfect passive participle variant vorsus, a, um "turned," and from there the 4th-declension noun vorsus, vorsūs, m. "furrow, the place where the plough turned around" and thence a unit of measure to delineate the edges of an ager, which was prevalent in Campania for whatever reason. It doesn't even have to be another Italic language at work here, as everything is perfectly explainable in Latin only. De Vaan (Etymological Dictionary of Latin and Other Italic Languages) notes that the ver/vor oscillation between present system and perfect participle seems to go back to Proto-Italic, but that in Latin in general, after 200 BCE, there is a general shift from vo- to ve-. We find Latin examples of both variations: vertex, verticis "whirlpool" and the more familiar modern variant vortex, vorticis, e.g.. In Oscan we find, as an epithet of Jupiter, *wersorei ("he who averts"), and in Umbrian, participles like vurtus or kuvertu "turned, averted". Proto-Indo-European *uert-e/o "to turn."

I see nothing in the Latin of dē rē rūsticā 1.10 about "vor" meaning 100 or "sum" meaning foot. Wherever you got that part is the problem. It's not in the original source, and the etymology of vorsus as a unit of agricultural measurement is not in any way a mystery.

Nam in Hispania ulteriore metiuntur iugis, in Campania versibus, apud nos in agro Romano ac Latino iugeris. Iugum vocant, quod iuncti boves uno die exarare possint. Versum dicunt centum pedes quoquo versum quadratum. Iugerum, quod quadratos duos actus habeat.

For in Further Spain they measure [land] with the iugum, in Campania they measure with the versus, in our parts, in the Roman fields and in Latium, they use the iugerum. They [in Spain] call it a iugum because yoked (iuncti) oxen are able to plow it up in one day. As for the versus, they say it is a "turned [measure]" of/for/at 100 feet (centum pedes), squared. The iugerum is what has two actus, squared.