A good 250gr at 1000fps from the sixgun is hard to beat and can be easily done in the New Vaquero, or even a bit higher up towards 1150fps. I would suggest finding an accurate load with 2400 around 1100fps maximum or 1000fps with Unique. The rifle will gain 200-300fps over those numbers. Even with Unique, my .38-40 and .44Mag loads run nearly 300fps faster than revolver velocities.
But, if it's just stamped "Vaquero", it's an "Old Vaquero, with the larger frame - and can accept loads that are fully the equal of anything the Model 92 will.
Actually the modern 1892 is about 20,000psi stronger than a large frame Vaquero.