I don't work with Adobe Media Encoder, but have you tried Debugmode Frameserver in Vegas Pro? If Adobe Media Encoder can read the signpost file that frameserver produces, you'll have the output of Vegas as if it was an uncompressed file, but in real-time.
Another way of working is to render a lossless (or visually lossless) avi file from Vegas and then bring that into Adobe Media Encoder.
Can you explain why? i.e., What is the "use case" for this? Is it for batch processing, or for a particular format you need that Vegas does not support?