If an exit is not an opportunity and you are stuck listening to this person, one strategy is find something, anything, in what they are saying that interests you and then asking them directly about it. Thus guiding them to talk about something you might find more interesting or useful. I allow people to talk about themselves if they say something interesting. I often end up learning something new. If they want to brag about themselves, i allow them that too and in fact take part in it if they want to feel good about themselves.
A narcissist will perhaps listen to you talk if you are saying wonderful things about them. So if you need them to listen to something you have to say, try sandwiching it between two compliments.