User blog comment:Kaido King of the Beasts/Chapter 813: Up in Vinsmoke/@comment-1595565-20160121175300/@comment-208.96.127.154-20160122002338

I think it`s the other way around. Someone from the Vinsmoke family is offering the arranged marriage in exchange for Sanji`s return. If that`s true, then that means they`re powerful enough that the bump in status is worth it for Big Mom.