temp[i][j] = (Grid[i][j-1] + Grid[i][j] + Grid[i+1][j] + Grid[i][j+1])/5

temp[i*PPE+j] = (Grid[i*PPE+j-1] + Grid[i*PPE+j] + Grid[(i+1)*PPE+j] + Grid[i*PPE+j+1])/5

:%s#v[^0-9](][)[a-z][^-+]#=substitute("1","*PPE+", "", "")#gc

:%s/vDzs][zeD]/*PPE+

[i][j]
[i+1][j]
[i][j+1]
[i+1][j+1]

[i*PPE+j]
[i+1][j]
[i][j+1]
[i+1][j+1]