How can I create a vector with specified NaNs' percentage?

I have a vector with 3000 data which 500 of them are real NaNs which I am not going to do any analysis for real NaNs.I would like to add 20% and 40% of the whole data as artificial NaNs (which should not overlay with the real NaNs) with different gap length from 1 to 30.

Any advice much would be appreciated in advance.