I made a similar cake for my cousin in March and a few weeks ago I made vanilla cupcakes with strawberry filling and strawberry cream "cheese" frosting for my sister's 21st. Here's what I learned and used:

I used recipes from Vegan Cupcakes Take Over the World. The golden vanilla cupcake recipe has a variation for lemony cupcakes. To make it more lemony added two tbsp of lemon zest instead of 1, and used 1 tbsp lemon extract and 1 tsp of vanilla instead of the amounts of almond and vanilla extract called for. I also reduced the amount of soy milk by 2 tbsp and added 2tbsp lemon syrup (like the kind they use in coffee houses) after the soy milk had curdled. This is a denser cake and not fluffy.

For the filling I sliced fresh strawberries, put a little sugar on them and let them sit to get the excess moisture out. Then I made the not-too-sweet blueberry mousse filling from VCTOTW (pg 153) using strawberries (you could use frozen too but I had lots of strawberries on hand).

For the frosting I used the Lemon Buttercream frosting from VCTOTW (pg 96) and used lemon extract instead of the vanilla called for, and also added a few more tbsp of lemon than called for but I was trying to make an icing rather than a frosting and it turned out well. Used extra lemon zest too.

Some things I learned:
When I made the lemon cake I multiplied the recipe by 4 and made two layers in 9 inch round pans. It made very think layers and I'd definitely only multiply the recipe by 3 next time for the round cake. Half the cake served 7 girls (the other half went home with the bday girl) in very large pieces.
Since you are making cake for that many people I am not sure how I'd suggest serving the cake. At first I was thinking cupcakes would be easier because you could already have individual servings but the labor involved in scooping out the middle of the cup cake to put in the filling would be really time consuming. When I made cupcakes for my sisters birthday I arranged them on a large platter and used scrabble letters to spell out the birthday message. I think with the amount of people you are having a 9 x13 square pan could be used (unless you happen to have a larger pan) and then you could use a serrated knife to slice the cake down the middle to create two layers (you'd have to make a few cakes I think). I suggest cutting a shallow hole into the bottom layer so the filling will fit in better and you can then put the top layer on without having the filling gush out the sides. I've noticed that vegan fillings don't hold their shape so well.

The icing and filling recipes in VCTOTW make about 1 1/2 times the amount you need for each cake recipe so you won't have to multiply the amounts by as much.

Oh, and if you are making multiple cakes, you should make the batter for each cake separately because vegan cakes really rely on chemical reactions to rise and if you are waiting to put a cake in the oven the reaction will have already started and you'll get a dense, short cake.

Hope this is helpful! Good luck!