why not use a immutable vector?

If we insert into leveldb quickly , the memtable will be also full quickly, but if the immutable do not write to disk, the main thread will be blocked to wait imm=NULL ;

So i want know why not use a immutable vector, when memtable is full ,it will be insert into immutable vector; therefore ,the main thread can not be blocked;

i know there is a background thread to dump imm, it schedule the task, so i think why not use this thread to solve the immutable vector?maybe set more background thread? just like main thread produce immutables and the backgraound threads to dump them.

thank you!