This has been a long standing issue with Vegas and MOV format. The best thing to do is one of two things:
1 - render your material to an intermediate format like cineform, XAVC-intra, or XDCAM (HD). Work in small groups of files.
2 - change the container type from MOV to MP4 using a utility app or FFMPEG. This process will not re-render the the media, it will only re-write the information from one container to another. Vegas will read your footage in .MP4, .AVI, or .MXF much better and not have the file limitation.