According to research work,post vasectomy pregnancy rate
after 5 yrs is nearly about 1% and it is regarded as least type of contraceptive
You have 3 young children and if you don't want to continue this pregnancy,then go for MTP
before 12 wks sothat risk is less.
You may take an additional contraceptive method in future like IUCD,condom or Emergency pill.
ALL the best.