Difference being, these were (presumably) not advance tickets, therefore break of journey was allowed.
Moreover getting to Vauxhall from either Brighton or Chichester is more difficult, involving a change, that may be why it was cheaper.
I've bought tickets to Victoria on the day (i.e. not advance), at a discount, then got off at Clapham Junction - and had the ticket scrutinised at the barrier. No problem.
Browsing through the discount advance tickets, I've been amazed at how cheap some of them are. Recently I bought an advance ticket, Leicester to London, for what I thought was reasonable (about £20). I now find you can get them for as little as £1! At that price, obviously a massive loss leader, I think it's reasonable that you should have to accept stringent T&C's. The important thing is to know what they are. But it appears, the provider has no obligation to put them in large print etc. etc... Caveat emptor...