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My understanding is that you want to control roll in a rocket, along with the other directions. An Atlas V has a single RD-180 engine but I didn't see anything about vernier engines. I guess the RD-180 has two combustion chambers and two nozzles, and if those nozzles can gimbal independently that would do it--point them in different directions and the rocket will roll. I suppose that must be the case, otherwise they'll be a little misaligned and you couldn't control it. But I couldn't find anything that explicitly states that.

RD180 model

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    $\begingroup$ Related: Atlas II used a set of hydrazine thrusters in the interstage for roll control after the vernier rockets were deleted from the first stage. $\endgroup$ – Russell Borogove Feb 16 at 1:01
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The RD-180 can gimbal each thrust chamber independently, allowing for roll control.

Each Chamber assembly is gimballed in two planes for thrust vector control. This feature is implemented through the innovative construction of the gimballing unit on each hot oxygen gasline. Strains developed by the actuator for chamber gimballing are confined structurally to the engine elements. This is an optimal solution that provides flight control not only in pitch and yaw, but also in roll direction.

(emphasis mine)

From Incorporation of RD-180 Failure Response Features in the Atlas V Booster Emergency Detection System

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    $\begingroup$ Beat me to the punch; slide 12 of this presentation confirms that the RD-180 provides roll control. $\endgroup$ – Russell Borogove Feb 16 at 0:43
  • $\begingroup$ @RussellBorogove looks like a good reference. $\endgroup$ – Organic Marble Feb 16 at 0:53

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