The vicar apparently "forgot" to introduce my reading at Grandma's funeral...?

I would have to say that it looks like your mother "forgot" to tell the vicar to include you in the service. It really sucks that she would be so petty during a time of grief when the family should be coming together and putting disagreements aside, but what's done is done. If you confront your mother, she's just going to deny it, right? And you'll get upset, and she'll come off as the victim. Instead, have the poem printed in the "in memoriam" section of the newspaper, dedicated to your grandmother. Hopefully, someday your mother will realize she's wrong, and you'll be able to put your relationship back together.