Help with vectorization

Hello everyone, 

I have a subroutine that definitely has the computational load that could benefit from a MIC. The subroutine is being used in a molecular simulation. The idea behind the subroutine is to determine which particles are within a certain distance, and for each particle create an array that says which particles are within that distance. However because I am only updating the array inside an if statement, the compiler sees a vector dependence and won't vectorize. I am wondering if anyone here sees a way to vectorize this subroutine.

program neighbor
  use ifport

  implicit none
  double precision, allocatable :: x(:),y(:),z(:)
  double precision :: dx,dy,dz,dr2
  double precision :: dens,vol,box,rcut2
  double precision :: x1,y1,z1,x2,y2,z2
  integer :: np
  integer :: i,j
  integer,allocatable :: numNeigh(:),vlist(:,:)

  np = 600
  allocate(numNeigh(np))
  allocate(vlist(1000,np))
  allocate(x(np),y(np),z(np))

  dens  = 1.0d0
  vol   = real(np)/dens
  box   = vol**(1.0d0/3.0)
  rcut2 = 2.50d0*2.50d0
  call seed(10)

  numNeigh(:) = 0
  vlist(:,:)  = 0

  !--- create arbitrary initial coordinates. Not important
  do i = 1,np
     x(i) = box*rand(); y(i) = box*rand(); z(i) = box*rand()
  enddo

  do i = 1,np
     x1 = x(i); y1 = y(i); z1 = z(i)
     !---would like to vectorize this inner loop
     do j=1,np

        if(i.eq.j)then
           cycle
        endif

        !---calcuate distance between particles i and j
        x2 = x(j); y2 = y(j); z2 = z(j)
        dx = x2-x1; dy = y2-y1; dz = z2-z1
        dr2 = dx**2 + dy**2 + dz**2

        if(dr2.lt.rcut2)then
           !---if distance is less than cutoff distance, update number of neighbors and store particle j in vlist
           numNeigh(i) = numNeigh(i) + 1
           vlist(numNeigh(i),i) = j
        endif

     enddo
  enddo

  stop
end program neighbor