No you don't have to use jelly in a trifle. In this recipe fruit compote replaces it. Vegan jelly is pretty hard to come by but it is possible to buy I believe. If you want to use jelly make it up as per the instructions on the packet then pour it over the cake pieces in the bottom of the bowl. Put in the fridge and allow to set before adding any more layers. If you do this you can omit the compote or add it in addition to the jelly.