Out of the box there is no way to do this.

I did at one time write a script in conjunction with a user-defined field for one of my clients to accomplish this. Essentially the user defined field would me a marker for the vendor being restricted. You would then set up a security role and give it to the people who would have access. When accessed through vendor maintenance or inquiry, the script would check to see if the user was a member of the role. If not the script would close the window immediately or hide certain tabs.

This also assumed that the person did not have access to reports that would disclose the information.

Here is a script for Terms Code. It would be similar:
retVal=0
terms=""

retVal=oBusObj.GetValue("TermsCode$",terms)

if terms = "00" then
retVal = oScript.SetError("You selected Terms Code '00'. Please select a different Terms Code.")

retVal = oScript.InvokeButton("fldr.pMain")
End if

In your case it would be PreWrite on the Vendor Number field. You would substitute the Vendor Number for the Terms Code

DSD has an enhancement "AP-1058 Security by Division" that we've used for this. www.dsdinc.com/.../