The rule is: 'that' defines, 'which' informs.
In other words, 'that' introduces information vital to the meaning of the sentence, 'which' provides useful supplemetary information.
Confusing the two can create ambiguity: 'The programme that VSO proposed was positively reviewed' suggests there were multiple programmes, and the thing that made this one different was that VSO proposed it. In 'the programme, which VSO proposed, was positively reviewed' , there could be one programme, or there could be several.