Best to use S-video whenever you can.
The
manual page 39 indicates that you can set the L1 output to S-video. You'd need a Scart plug with a S-video socket, like one of these:
Scart connector.jpg
Although you say the VCR has an S-video connection? If that is the case, ditch the Scart and sue the S-video connection.
ES-15 will definitely help.
Connect the VCR to the ES15 with S-video (or you can try with the RCA cables, it will still be a big improvement), then S-video from the ES15 to the Roxio (once again, you can use RCA, but S-video is better).
See what that does to the quality. Then you can decide if you want/need to go down the PC rabbit hole.