Solid rockets have very high "density specific impulse," that is, they produce a lot of thrust per unit volume, so an equivalent liquid stage of the same diameter would be somewhat longer, but not prohibitively so.
There are a few extant liquid-propellant engines that produce similar thrust to the Vega first stage that could be used. The hydrogen-oxygen RS-68 produces sufficient thrust in a 2.43m diameter engine, but due to hydrogen's low density, you'd need big propellant tanks, yielding a very long and skinny stage. The BE-4 and Raptor are also in the same general thrust class.
The very largest solid rocket boosters -- the shuttle's SRBs, and the similar 5-segment SRBs intended for use on the SLS -- are more powerful than any single liquid propellant engine built thus far (such as the 4-chamber RD-170 and the Saturn V's F-1), but much larger liquid engines have been considered.
Among engines in production today, the RD-191 would probably be the best candidate for a liquid engine to replace the Vega P80 first stage. At the same mass as the solid first stage, it would have slightly initial lower acceleration off the pad, which would be balanced out by the increased mass-specific impulse (311 seconds at sea level, versus the P80's 280). I estimate the stage would be about 14 meters long, just a little longer than the P80 solid stage.
Vega is quite small, as orbital launchers go, but at that scale, liquid engines are competitive with solids by mass and volume.