Stream Ciphers generate a long binary key that looks totally random, but in fact is generated from a short key known as seed.
The seed is held by both writer and reader, so both can generate the same Vernam key.
The most common stream ciphers are known as permutation stream ciphers. They rely on a specific order of the 2^{s} permutations of an s bits seed, S, where each permutation contributes a bit to the generated Vernam key.
The maximum length key of, say, a 32 bits seed is 4,294,867,296 bits long key. 
