Quorum? There is no voting for a soft fork. Miners either upgrade their client or do not. In a soft fork, all old nodes can see the new blocks, can still have transactions sent to addresses that they recognize, and can spend their existing coins.
Yes there is.. Soft-forks only reach super-majority (and the feature switched on) if the block versions produced by miners reaches reach a number indicating the change. 95% of 1250 concurrent blocks is the exact mechanism - miners have full choice over what software they run.