A hydroprocessing catalyst for treating a heavy hydrocarbon feedstock having significant concentrations of vanadium, wherein said hydroprocessing catalyst comprises: a calcined particle comprising a co-mulled mixture made by co-mulling inorganic oxide powder, molybdenum trioxide powder, and Group VIII metal particles and then forming said co-mulled mixture into a particle that is calcined to thereby provide said calcined particle, wherein said calcined particle has a pore structure such that at least 23% of the total pore volume of said calcined particle is in the pores of said calcined particle having pore diameters greater than 5,000 Å and less than 70% of the total pore volume of said calcined particle is in the pores of said calcined particle having pore diameters in the range of from 70 Å to 250 Å, as measured by mercury penetration; and wherein said molybdenum trioxide powder used to prepare said co-mulled mixture is in a finely divided state of particulate molybdenum trioxide either as a finely divided powdered solid or as a suspension wherein said particulate molybdenum trioxide is of a particle size having a maximum dimension in the range of from 0.2 to 150 μm; and wherein said co-mulling step is conducted such that said co-mulled mixture has a pH that is maintained in the range of from 6 to 9; and wherein the calcining of said particle is conducted under a controlled temperature condition at a calcination temperature in the range of from 482°C (900°F) to 787.7°C (1450°F) for a calcination time period so as to provide said calcined particle having said pore structure; and wherein said calcined particle has a molybdenum content in the range of from 2 weight percent to 12 weight percent with the weight percent being based upon the molybdenum as MoOs and the total weight of said calcined particle; wherein said calcined particle has a nickel content in the range of from 0.2 weight percent to 6 weight percent based on the nickel as NiO and the total weight of said calcined particle, and a cobalt content in the range of from 0.2 weight percent to 6 weight percent with the weight percent being based upon the cobalt as CoO and the total weight of said calcined particle; wherein the surface area of said calcined particle is greater than 250 m 2 /g. wherein said molybdenum trioxide powder used to prepare said co-mulled mixture is in a finely divided state of particulate molybdenum trioxide either as a finely divided powdered solid or as a suspension wherein said particulate molybdenum trioxide is of a particle size having a maximum dimension in the range of from 0.2 to 150 μm; and wherein said co-mulling step is conducted such that said co-mulled mixture has a pH that is maintained in the range of from 6 to 9; and wherein the calcining of said particle is conducted under a controlled temperature condition at a calcination temperature in the range of from 482°C (900°F) to 787.7°C (1450°F) for a calcination time period so as to provide said calcined particle having said pore structure; and wherein said calcined particle has a molybdenum content in the range of from 2 weight percent to 12 weight percent with the weight percent being based upon the molybdenum as MoOs and the total weight of said calcined particle; wherein said calcined particle has a nickel content in the range of from 0.2 weight percent to 6 weight percent based on the nickel as NiO and the total weight of said calcined particle, and a cobalt content in the range of from 0.2 weight percent to 6 weight percent with the weight percent being based upon the cobalt as CoO and the total weight of said calcined particle; wherein the surface area of said calcined particle is greater than 250 m 2 /g.