What is difference between nimitta and vitaka and their relation to manasikara?

I learned many meaning of vitaka.. Like initial thought, or thought seed or directing mind toward object.. Whereas vicara is continuity of thinking about that object. Similarly nimitta is arising of some mental sign/clue etc. Like I remember today's event, or I heard something etc. There is sanna also in conjunction with nimitta... I want to know whether vitaka and nimitta same or different? If different I want to know practical understanding of the difference between them ?