I COMPLETELY understand your feelings... Cake- $25.00, wedding cake- $100.00..........Mentioning "wedding really" does quadruple the price.
This is a tricky situation... Perhaps you can explain it exactly as you said- "I'm looking to have a large family dinner where my fiance and I will exchange vows." Hopefully that shouldn't inflate the price... You already have your own vendors for a minister, flowers, etc...
You may want to try a little 'white lie'-
"My fiance and I eloped abroad and are interested in having a little reception where we renew our vows in front of our friends."
or
You could book the party as a dinner........... and then later explain that you got engaged and would like to have a small ceremony while you already have everyone at the dinner...
Or- you could have a friend or your fiance call and inquire about what you want... Then at least you have an answer as to whether or not you need to be 'creative in explaining the situation.
Good luck! I hope you get what you want without an inflated price.