Burlington’s Flynn Theater is a perfect venue for a modern-day rock opera, and that is what fans expected last Sunday. The Decemberists were set to perform their opus “Hazards of Love” in its entirety, with two female guest vocalists and the band’s standard cast of multi-instrumentalists. But an airport delay left one of the guest vocalists, Becky Stark, elsewhere and the band was forced to play what lead singer Colin Meloy dubbed “a mixed bag.” A Mount Mansfield High School junior, Alan Hefferon, wasn’t disappointed by the last-minute shift. “The mix of old and new was much more scintillating than the “Hazards” set would have been,” he said. Opener Laura Veirs & The Hall of Flames played a set of cutesy but accomplished pop songs before fog covered the stage and huge swaths of net fell from the ceiling.  The combination of net and colored lights created a lovely ethereal ambience.The Decemberists’ first set showcased the depth of their discography, with songs from all but one of their full-length albumsWitty banter and good vibes within the band kept the audience engaged and entertained between the well-executed tunes.Second guest vocalist Shara Worden did make the show, and she brought the concert to the next level. Her vocal performance and exhilarating stage presence demanded attention and awe.Tracks from “Hazards” showcased her theatricality and stage presence, not to mention vocal prowess. More than halfway through the first set, new single “The Rake Song” got the crowd out of their chairs and dancing for the first time. “The Flynn is a lovely place, but this shouldn’t have been a seated show,” Plattsburgh bank teller Jenny Shafer said. Perhaps the “Hazards of Love” set would have been better suited to theater seating, but the venue was inadequate for Sunday’s performance. The Decemberists ended their second set with a cover of Heart’s “Crazy On You” featuring Worden on vocals. The encore featured a new song played by a solo Meloy — a tender, sad acoustic number. UVM freshman Tom Campbell summed the show up well. “The hard rock of ‘Hazards of Love’ and the melodic beauty [of the band’s other songs] made for an experience I won’t forget.”