In Essen is also a branch of the "Veganz" supermarket chain. It´s the ideal place to get some snacks for the convention. It´s close to the u-bahn station "Berliner Platz". And you can take the U11 directly to the Messe from there
I can definitely recommend the Zodiac, both their pizzas and their restaurant are excellent. The restaurant gets busy during Essen so you probably need to book.
There is a branch of the Italian chain Vapiano at Rathenaustrasse 2 which is about 5 mins walk (max) into town from the Hauptbahnhof. They speak English and prepare pasta and pizza any way you want it. I haven't been since I became vegan but I dont think their pasta has egg in it. You also get complementary very tasty ciabatta bread.
We're a group of vegans heading to Spiel for the fifth time. Theee is a great restaurant at Kennedy Platz as well. I can see if I can find the name, but we usually go there so we just know it's location