One time vendors can only be used for specific vendor account groups. Their
is a flag for each account group in configuation for this. If one does not exist
you will have to create a new vendor account group.
To create a One Time Vendor use XK01 and select the OTV account group.
When creating say a PO use your OTV account the transaction will prompt you
for name, address etc (unlike a normal vendor).
You experienced a problem with OTV's whereby users couldn't be bothered to
create permanent vendors and just used the OTV account, so beware of that.