I'm not getting married or engaged anytime soon lol....but I have been to one wedding this year and am going to another one in a months time, and both catering companies have been extremely accommodating to my boyfriend and I's vegan diet.
For the wedding we have already been too they made a vegan option for each entrée. For starters we had mini bruchettas, then we had pumpkin and basil risotto, for the main they made Moroccan eggplant and pumpkin cous cous, and for dessert was beetroot and blueberry crumble! They made all this for us and we were the only two vegans there! It was amazing.
The catering company for the other wedding which we are going to next month basically have told us that they have done a few exclusive vegetarian weddings in the past (even ones where none of the guests were allowed to wear leather or animal products!) and will be able to make us whatever vegan dish we want.
You pay a lot of money for catering a wedding, in fact most of your wedding budget will probably be spent on food - so for the money involved the majority of catering companies will be more than happy to make a vegan menu.
For the engagement party they rented out a nice function room at a bowling club and hired a catering company which did finger food. What I would do for a vegan engagement party would be too make all the vegan finger food myself and then freeze it. I would then pay a comapny to heat up the food for me on the night, and serve it to the guests
Wow that turned out to be a long post lol sorry. Good luck to all of those getting married.