The course of operatic true love never did run smooth. When the Roman general Lucinius returns in triumph from the Gallic Wars only to discover that his beloved Julia has been forced into taking the veil as a Vestal Virgin, the future looks distinctly rocky – and that's rocky as in Aida. Gaspare Spontini's La vestale was a huge hit when it was composed in Paris in 1805, at the height of Napoleonic power, but it has become a rarity now. It has weaknesses, but in last night's performance by La Monnaie, those don't prevent a thoroughly engaging evening.

La Monnaie's main house is undergoing extensive renovation works, so their season is being spread around various venues in Brussels. La vestale is in the Cirque Royal which, as the name suggests, is basically round. The first surprise, as you enter, is that the orchestra is in a semi-circular space nearly on the level of the stage, with conductor Alessandro de Marchi at the stage end, facing the audience as he conducts the overture. Seated close to the timpanist in row D, I felt very much part of the orchestra.

The overture is very exciting in a Beethoven-like way, and de Marchi conducts with verve. Singers file in and out of the semi-circular space between orchestra and audience, until we see the figures of Lucinius (Yann Beuron) and his faithful retainer Cinna (Julien Dran), who fill us in on the back story. We are very much in the flavour of French classical drama: Étienne de Jouy's verses may not be up to Corneille or Racine, but they are elegant, and Beuron and Dran set the scene by giving us pin sharp diction, Beuron singing the hero's part with a warm, open tone.

The opera is a star vehicle for Julia: a big dramatic role where the singer must carry the audience through every phase of inner conflict as Julia fails to reconcile the irreconcilable: her vows (however unwillingly sworn), her love for Lucinius, the mortal danger they face if that love is to be requited. Alexandra Deshorties puts in a bravura performance, clear-voiced, sweet-toned or screaming according to the mood of the moment, negotiating Spontini's tricky intervals with ease, and utterly inhabiting her character.

Her antagonists have easier roles to sing, but if anything, they're even stronger vocally. As La Grande Vestale, Sylvie Brunet-Grupposo makes you sit up and listen from her first note, exuding authority in a powerful mezzo with a creamy tone in her middle and low range. Jean Teigen, the High Priest of Jupiter, is stentorian. Both pronounce their lines with the clarity of poetry that characterises all the singers.

The music thrills most in the big choral numbers: the end of Act II, when Julia is condemned in front of the baying crowd, is electric, as is the moment when she is punished by being buried alive. The orchestral score is superb, and De Marchi winds orchestra and chorus to fever pitch for these big climaxes.

Éric Lacascade's staging is sparse – hardly anything in the way of sets, just a few significant props, most notably the hexagonal prism that serves both as Vesta's shrine and Julia's place of punishment. Marguerite Borda's costumes are plain, effective and timeless: this is an opera that could be set in any age. Lighting is subdued. The acting and stagecraft isn't perfect – some of the movement isn't sharp enough, and there's a truly cringeworthy moment when the flame which is supposed to be ignited by a lightning strike from the gods is lit by a chorus member running on with a cigarette lighter – but broadly, speaking, the acting supports the drama well.

So why has La vestale been relegated to a mere curiosity? Compare it to Norma, another opera where a Roman general gets a priestess into trouble and another Callas star vehicle in the 1950s: La vestale has a story that's every bit as powerful, is more classically dramatic, relies less on implausible plot coincidences, and has a poetic libretto that's just as elegant and far richer orchestration. The main difference is in the quality of the vocal writing: what La vestale lacks is a knock-your-socks-off aria of the quality of Casta Diva. And the other big defect is the deus-ex-machina ending: at the highest moment of tension, the gods intervene with a lightning strike to ensure that our lovers are forgiven, whereupon a tackily cheerful divertissement follows. To our modern ears, the intensity of the preceding tragedy is utterly subverted.

These defects are real and important, but shouldn't be taken out of proportion: this was an opera that entertained thoroughly with some great drama and superb playing and singing, an evening of far more than mere historical interest.