Tunable VUV laser (ω4=2ω1+ω3) in the range of 124.80-125.06nm is gene-rated by four-wave sum mixing in Hg vapor with Ar as buffer gas, when one of the pump beams (ω1) is tuned exactly to 61S0—71S0 two-photon resonance and another pump beam (ω3) on the blue side of 71S0—91P1 transition. The experimental results are analysed with the small signal four-wave sum mixing theory. The factors affecting the best phase matching condition and the methods to expand the tunable range of VUV output in the weak pump case are discussed.