Complex sentence: Soon after his arrival he was sent to the Illinois mission, for in 1728 he seems to have been at Post Vincennes, "the fort on the Wabash", which was established about that time.
Split into simpler sentences: Soon after his arrival, he was sent to the Illinois mission. In 1728 he was recorded at Post Vincennes, "the fort on the Wabash" (River), which was established about that time.