You just never know who you’re going to see at Luck Reunion. The tenth rendition of Willie Nelson’s annual music festival, held on his storied ranch in Spicewood, teased a surprise guest in its lineup and then overdelivered, with the same nonchalance and ease with which the festival—and Willie—does everything.How many ways can you substantiate a good vibe? At Luck, that vibe is evidenced by the way strangers make room for one another in front of a stage, and by how unhurriedly they move through the movie-set “town” (originally built for 1986 film Red Headed Stranger). The festival notoriously releases its schedule the morning of the event. The kind of plotting and planning that would have you at one stage for exactly eight minutes before…