It's never too early to start making a list of things your vendors should know. Things should be jotted down done whenever you remember so that you will not forget anything.
Things like food allergy among your guests should be communicated to your caterer or reception hall right away and remind them when you confirm the number of guests you have as your wedding day comes closer.
If you have an open bar and your Uncle Bob tends to overdo the drinks at events like these, give the bartender a heads up.
Whoever is making the table arrangements, inform them of any sensitive relationships. If there are any other relationships that should not be brought to attention, inform those who are involved.
These are little details of the wedding that can be overlooked but are important and might have a negative impact on your big day.