If the people we're gathering intelligence on (the "targets") knew how we are collecting that intelligence (the "vectors"), then they'd be able to avoid being spied on.

Also, sometimes the vectors themselves, if exposed, would cause diplomatic incidents -- human intelligence (spies), placed within a foreign government, or illegal means of collection (black bag job breakins, tapping undersea cables in national waters, ...).

Ah, so by "if this were not so" he meant "if they weren't protected"? I read it as "if they didn't have to be protected", which is why I was confused.

If you knew your phone was tapped, you could stop using it.