Diminutive midfielder Valbuena struck from inside the box after six minutes in a stuttering performance by the Provence side, who have 71 points with four games left.
Second-placed AJ Auxerre, on 66 points, won 3-0 at Toulouse earlier on Sunday thanks to two goals from Poland striker Ireneusz Jelen.
Marseille and Auxerre meet at Auxerre's Abbe Deschamps stadium on Friday.
"That is why we had to win tonight," Marseille coach Didier Deschamps, the last OM captain to claim the league title, in 1992, told reporters.
"We could have killed it in the first half. We suffered after the break, we had some problems keeping the ball. There were some hot moments but we won, that's what matters."
Marseille got off to a perfect start, with Valbuena beating Jeremie Janot with an angled shot from inside the box after being set up by forward Brandao.
The home side did not ease up and came close to a second after 20 minutes when Janot tipped a Mamadou Niang powerful header over his bar.
The St Etienne keeper was helpless at the half hour with another Niang header but the Senegal striker's attempt struck the post.
JELEN DOUBLE
St Etienne's attacks were easily thwarted by the impressive Marseille centre-back Stephane M'Bia but Marseille's game deteriorated after the interval as Les Verts stepped up a gear.
St Etienne had a goal disallowed shortly after the hour when Emmanuel Riviere fired home but the striker was ruled offside.
A few minutes later, a Yohan Benalouane attempt hit Steve Mandanda's post as Marseille eventually held on.
Auxerre's victory was far more convincing.
The Burgundy side, who claimed their only top-flight crown in 1996, played solidly in the first half and came out stronger after the interval.
"We had five very tough minutes, when we could not get out of our half of the field and barely managed three passes in a row," Auxerre captain Benoit Pedretti told French TV Channel Foot+.
"Then, we waited for counter-attack opportunities and set-pieces. We made the most of our chances."
Pedretti opened the scoring with a fine dipping free kick two minutes into the second half and Jelen doubled the lead with a delightful 25-metre shot on 67 minutes.
Jelen put the result beyond doubt with 11 minutes remaining, latching on to Valter Birsa's cross from the left.
Elsewhere, Lens were held to a 1-1 home draw by neighbours Valenciennes.
