Viterbi decoder on a bit stream where the preamble is also coded
Is the 500-bit encoded message part of a continuous stream being decoded by the Viterbi decoder, or is it a 500-bit packet that is decoded individually?   If part of a stream, then decoding continuously and searching for the preamble indicates the beginning of the message.   If packetized, then the decoder memory needs to be managed so that the first decoded bit location is known.   This will be dependent on the decoder implementation.