How can Vincent appear in the scenes at the end of Pulp Fiction

Pulp Fiction uses a 'non-linear narrative', i.e. its a story where the scenes are not set in chronological/time order.

The scene where Vincent appears at the end, is (in real time) before the scene where he is killed.

You can also find an excellent commentary on this at the following Tarantino Wiki.

The “final” scenes from Pulp Fiction relate the story of Butch and Marsellus and the events in the pawn shop. This is demonstrated by Julius and Vincent arriving at the bar with the briefcase wearing the casual clothing that they changed into at Jimmy’s house at the begin of Butch’s tale. The events in the restaurant with Honey Bunny (the holdup) actually take place before Vincent is killed and thus are the next to last if the film would be reconstructed in chronological order.

So the film’s “chronological” order is:

1. Butch receiving his father’s watch (The first portion of Butch’s story) when he was a child.
2. Vincent and Julius retrieving Marsellus’ briefcase
3. Vincent and Julius going to Jimmy’s house after they accidentally kill Marvin.
4. The robbery in the diner (the opening and closing scenes of the film)
5. Vincent and Butch arriving at the bar. Butch being instructed to throw the fight. Vincent and Butch clashing
6. Vincent and Mia Wallace’s date/Mia’s overdose
7. Butch not throwing the fight and escaping
8. Butch returning to his apartment and killing Vincent Vega
9. Butch and Marsellus and their troubles in the pawn shop (The “true” end of the film)

As noted in the other answer to this question, the non-linear narrative makes it difficult to discern what is happening if you don’t watch the film closely.