i want to surprise my partner with a destination honeymoon

Are you planning on spending a significant amount of time in Vegas after the wedding? If so, you could treat that as your honeymoon. Then you could plan a “second honeymoon” for your one year anniversary.

Are you sure that a surprise trip is something your FI would enjoy or appreciate? For me, half the fun of a trip is in the anticipation. I'd also want the heads up for planning and packing purposes. Going somewhere else after Vegas is entirely up to you ( and in his place I'd say your FI) to decide together.