To withdraw n BTS from a vesting balance with vesting period T you need to accumulate n*T in coin-time.
If you have a balance of n1 that has already accumulated n1*t1 coin-time you can withdraw n1*t1/T right now (for t1 < T), or you can wait another (T-t1) and then withdraw the full balance.
If at this point the balance increases by n2, you must accumulate additional n1*(T-t1)+n2*T coin-time before you can withdraw the full n1+n2. Obviously, this takes longer than (T-t1) but shorter than T.