Keith is saying, change this:
varTo = DLookup("[VendorEmail]", "[Vendor]", stWho)
To
varTo = DLookup("[VendorEmail]", "[Vendor]", stWhere)
Additionally, your stWhere is messed up, so change
stWhere = "Vendor.VendorEmail = " & "'" & stWho & "'"
To
stWhere = "[VendorEmail] = '" & stWho & "'"