As MSW noted, the idea of Anakin Skywalker’s lightsaber, i.e. the one Rey delivered back to Luke Skywalker at the end of The Force Awakens, being locked with in combat with a lightsaber being powered by Darth Vader’s kyber crystal would be an amazing moment.
Perhaps he kept another laying around, and it’s this one that Luke disassembled to procure the red kyber crystal.
However, that doesn’t necessarily mean that that was Vader’s only lightsaber.
Keeping in mind how Kylo Ren will be taking steps to emulate granddaddy Darth Vader in Star Wars: The Last Jedi, that crystal might be too good of a souvenir to pass up stealing if he spots it, even though he already has his own lightsaber.