Don't expect too much. This is a low cost scope. The rear cell is a poor construction. While the mount rotates the scope over the course of the night gravity pulls so strong that you see several pixels movement between 5min subs when guiding with a guide scope. So what you call a perfect collimation on the table will look completely different if you point the scope up or turn it upside down.
Unless you use a flattener the stars in the corner will always be oval. The field is curved. The best you can get is either the short or the long axis of the elliptical stars point to the center. For a field you need a flattener.
I'd also like to point out that the engraving on the secondary is not exactly the center of the curvature. The procedure you use relies on that. The procedure described in the pdf by DSI does not.
After learning the basics with the RC6 I upgraded to the RC8 carbon which is a bit better mechanically.
If it is about testing the collimation only you might want to use PI's FWHM eccentricity tool. If the plot is somewhat symmetric and best in the center you are in the ballpark.
Here is what my RC8 looks like. Scale is 0.733 arcsec/pixel using the TS 0.66 reducer.
Note that 0.5 is the limit of what appears round. So I always crop the image. If I have a larger target I use a flat field refractor with a shorter focal lenght. The RC is really a small target scope unless you use a flattener.
I tried your vega image but it does not contain enough stars to generate a map.