in a pop wrangle, set your first input binding to your vel volumes SOP path, then:
// using VDBs
vector v = volumesamplev(0,0,@P);
// or using Houdini volumes
float vx = volumesample(0,0,@P);
float vy = volumesample(0,1,@P);
float vz = volumesample(0,2,@P);
vector v = set(vx,vy,vz);