If i donate in installments, for example $10 each month. Then at the end of 5th month will i get VIP Rank? Or if i donate $15 and a bud in next month. Will this be considered $50 donation for VIP rank?

Yes, it's cumulative. You can donate $10 now and $40 in 6 months and you'll still get the VIP rank.

For reference, here's my history: