I need opinions! Birthday party/wedding combination?

I am 44 years old, and finally met the love of my life a year and a half ago. He is 48, we have both been married and divorced and each have grown children. We are absolutely perfect for each other, and have even bought a house together already. We are going to get married at some point, but do not want to make a big fuss with a wedding, as we have both already had the 'big' wedding. Anyway, his dad and I are very close and my fiance and I are throwing a 75th birthday party for him this summer. The other night my fiance asked me what I thought of surprising everyone and having a quick ceremony during dad's birthday party since we will have so many family and friends around. We do not want to take anything away from his dad's celebration though. We know our friends and family - and especially his dad - will be thrilled we are getting married - but we are worried that it may somehow take away some of dad's spotlight and/or turn into a party for us. That is one of the reasons we would keep it a complete secret from all of the guests - they will be there to celebrate dad's birthday - and just want to take a few minutes out to exchange our vows. Based on this info - what are your thoughts??