Vratams are for us only and not for Perumal. One can offer festive food to Perumal and partake it. It is up to the individual whether to fast or accept food. If they feel Vratam is more important than the festival then they can observe vratam or if they feel the festival is important, they can offer special food during that day and partake it, but broken rice should be used for cooking.
Observing vratam on a Ekadasi is considered more sacred than partaking festive food.