Plan B Effectiveness & Vasectomy Concerns: Seeking Expert Advice

Question
My GF and I had unprotected sex and I wasn't able to pull out and I ejaculated within her. She was on day 4 of her cycle. She was barely bleeding the night before. So out of precaution we took plan B less than an hour after I came inside her. My question is this. Is there a good chance that it worked? Another bit of info is that I had a recent vasectomy. I had one test done almost 2 weeks before which had no sperm but I took a test 6 days later and they saw ONE live sperm. I know we should have been more careful and we had a moment of weakness. I asked her how many days are her cycle but she counts from when her period ends but when she tells me her last period was in March and when I see when she had it in April and I count from when her period began its more like 28 days.

Answer
There is pretty much no chance that your girlfriend got pregnant from the situation you've described.  A woman will rarely ovulate near day 4 of her cycle.  You apparently don't have enough sperm to cause a pregnancy.  And plan B will prevent more than 90% of pregnancies that might otherwise occur.  She's not pregnant.