Is it necessary to wash the clothes and vessels of one who enters Islam?

That is not necessary, because the more correct view is that
the najaasah of non-Muslims is intangible, not tangible, because the Prophet
(peace and blessings of Allaah be upon him) and his companions did
wudoo’ after drinking from the leather vessel of a mushrik woman, and
because Allaah has permitted us to marry the women of the People of the Book
(i.e., Jewish and Christian women), which implies touching them. And because
Allaah has permitted the food of those to whom the Book was given, which
also usually implies that they must have touched the food. 

Based on that, if a man becomes Muslim he does not have to
wash his clothes, bedding and so on, unless he is certain that some najaasah
(impurity) got onto them and they have not been purified from it. End
quote. 

Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him).