0.25 - 1% If u are asking having unprotected sex after a vasectomy. Men should not fail to have their semen checked post vasectomy. Above risk is after a simple vasectomy, also slight chance of developing a seroma which very rarely can lead to recanalization. Doesn't occur if 2 proximal ends of vas placed in one hemiscrotum and 2 distal in other hemiscrotum. Chances after vas reversal are approximately 50%.
Vas Reversal. Depends on time since vas was done an expertise on the doctor reversing it...That urologist can give you good stats...You want his personal stats not the stats for the country.
About... After becoming a sperm-free man in semen from vasectomy, less 0.5-1 % of them may have post-vasectomy recanalization (or rconnection) leading to occasional pregnancy. Remember: nothing could be 100%.