It's hard to give advice for arguments without knowing your exact reason for not vaccinating or delaying (i.e. ingredients, potential side effects, did someone you know suffer a severe reaction, no fear of the VPDs, all of these reasons? etc). With that said, there is a reason you are not comfortable with vaccines at this time. You shouldn't have to explain yourself any further. I would just say you have done your research and do not feel comfortable with this, this, or that, right now, for whatever reason you are not vaccinating. End of story. If you want to get into a hot debate about it, then I would surely pull out your research and brush up on what you have learned before hanging out with these people again 
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If not, I would leave it at that and just tell them how you feel, for example, that you feel they are unsafe and don't want your child subjected to them, etc.