The vouchers, even the Southern ones can be used on any line in the country (apparently) or exchanged for cash as mentioned above. And it doesn't have to be a season ticket to claim, any ticket more than 30 mins late will do.
Train vouchers. As I have a season ticket they are useless to me. However, as much as they say on the back of them "These cannot be exchanged for cash", they can, and I have done so, and Southern have confirmed that you should be able to.