Neither safrole or isosafrole will decompose if distilled without vacuum. If you leave the vacuum out you will also get a lot better fraction separation, as that is one of the trade-offs with vacuum distillation - the boiling points of different substances come closer together. Also, you are more likely to experience insane bumping of the liquid under vacuum.
I have no idea about which impurities you are talking about which would cause extensive decomposition at atmospherical pressure distillation. If you have any substance to that argument, Mr. MDMA_AcTIvEsTS, please step forwards with them.