You must convert the uniqueid to characters

select *, isnull(vendeurs.cVendID,space(10)) as cVendeur 
from JobH left join vendeurs on JobH.cVendId = CAST(Vendeurs.nID AS CHAR(36))

but if you replace JobH.cVendId empties with NULL ( this is the correct choice )
this conversion is not need

Fabio