You won't be sterile straight away. There will be residual sperms left in the upper part of vas deferens, beyond the vasectomy site.

It usually takes some time following the procedure for all the sperms to clear from your ejaculate. You will need to perform a semen test 3 months from the time of your vasectomy. Please use some alternate method of contraception until it is confirmed that you do not have any live or dead sperms in your semen sample.