RE: How to set Vendor Credit Limit......
Under Miscellaneous Details, you can define Vendor credit Limit.
The maximum amount that the organization can have as an outstanding balance for the selected vendor account. This amount is always stated in the currency of your organization.
The account balance is calculated according to the selection in the Credit rating area of the Accounts payable parameters form. If an account balance becomes larger than the credit limit that is specified here, no additional purchase orders to the selected vendor can be posted. If this field is blank, or if you entered 0, a credit limit check is not performed.
On AP Parameters, there is Credit rating Tab
References
technet.microsoft.com/.../aa592162.aspx