If ([Vendor Name] <> [VN]) then

You are correct in using <> as meaning not equal, but there is a catch here.

Because you are using what appears to be a control or field value [Vendor name] is a variant variable.

If either [Vendor Name] or [VN] has no value, that is null. Then you logic test will fail. You could either test for null or use the NZ function to get around this.

Also the comparision will be insensitive to upper or lower case.