About 30 years after a vasectomy, what is the chance of a reversal and fathering a child again?

Depends. I need to know how old you and your partner are. A vas reversal after 30 years is possible but if your partner is over 35, i likely recommend ivf with sperm extraction.
Chance is high. Options: vasectomy reversal or surgical sperm harvest from the testes and using the sperm for ivf and intracytoplasmic sperm injections into eggs. Chance depends on age of female partner, condition of the vas, experience of microsurgeon, presence of antisperm antibodies. Chance of success of reversal is lower with longet time after vasoctomy about 25% after 25 years.