So there's been a fair amount of speculation about what Vito told Sanji, the normally indomitable badass and hardline strategist, that got him to go along with the summons for his impending marriage. Some think it's related to his family, others believe they've got dirt on Sanji or one of his friends, and still others propose that it's something else unrelated to those two. However, given what we know so far about Sanji, his family, and Big Mom, I think we can deduce precisely what the chefmeister was told that made him so compliant, if not the wording itself; no harm in doing so, considering that we probably won't find out for certain for 5 more years or so.
First off, let's look at the marriage. There's no way in hell that the Vinsmokes are doing this for Sanji's sake; it's far more likely that this is a political maneuver of sorts for a family heavily involved in the underworld, one that is completely fine with allying with a Yonko somehow, and one that sees this as a good way of securing that alliance. God knows divorce probably doesn't even exist in this world. Anywho, this means that this Purin lady may not be good-hearted or good-looking (considering who her mother is, neither would surprise me), and we all know that Sanji likes 'em hot, so if she's anything like her mother the Vinsmokes would want to convince Sanji some way or another to marry her despite his heavy reservations. Whatever Vito said would take this into account.
Secondly, think about Sanji himself. Woman-chasing nature aside, he is incredibly protective of women; within the last two arcs, we've had him declare that 1. he knows the sound of a woman's tears, and 2. he is willing to forgive a woman's lies, all at his own expense. There's no denying that whatever the situation, as long as the woman is reasonably attractive and good/neutral, Sanji's willing to drop everything just to help her out. With this in mind, it's entirely likely that Purin would be put in some sort of distress if Sanji didn't agree to this marriage, and it would weigh heavily on his mind if he let this happen, so he's set out to at least stop the marriage while protecting her.
With these two aspects of the story in mind, I think it's safe to conclude that Vito's whispered message to Sanji was: