Your dress sounds similar to mine and I'm going on the 20th on the Sky so I can let you know if you haven't left yet. From what I'm seeing anything from short to bridesmaid gowns are ok on formal night. (I have a black satin sheath with spaghetti straps).
Bought Hubby a gold-ish vest to go with his tux so he can dress up too. It's our annual -formal portrait!