Both countries have long, independently developed vodka traditions and each treats the drink as a national invention; the earliest solid written record is Polish (1405), but Russian state production and taxation records are also very old and the surviving documentary trail on both sides is incomplete.
What would resolve this A dated written or archaeological record of distillation for drink, rather than medicine, that clearly predates 1405 in either country would settle which tradition is older; short of that the question is likely to stay a matter of national narrative rather than documentary fact.
OpenCulinary historyVodka (Wikipedia)