There has been lots of excellent advice in this post! I noted what DJ noted that the KS is hitting the pipe. It would be a good idea to put the longer spline in and give it just enough to clear the pipe.
I do agree that it is best to ignore where the pistons are as the "good" technique works no matter where the flywheels ar sitting. However, if you were watching me start a twin you might think I am positioning the flywheels. Instead, I am prodding the engine to get the cams to push the valves open to ease the strain on the whole decompression system. Once the valves are open the decompression lever can be held to hold them there.
I know that we describe releasing the decompression lever at "half way down" but I think that is easier to do once you have gained a little confidence. From looking at the video, I might have said "release the lever when you are done with the kick." It seems that the tendency when practicing the technique is an "early release", not that I would know anything about that...I've just heard stories about it. But, if you get any compression on the way down, the kick will not start the engine. Don't waste the energy, start over.
I did not see "indexing" mentioned. Sometimes rotating the KS a tooth or more on the spline (counter clockwise) will let you use the full force of extending your leg rather than terminating the kick stroke when your leg is still bent. It is usually not an issue on the twins, but it can be on the much taller Eglis.
I was taught never to start a Vincent on a stand. I always followed that advice and, after doing it so often I found it very awkward to start one on a stand. Don't get me wrong, I don't want anyone tipping over at this point, but it is very hard on the rear stand the stock D stand. The Dave Hills stand would certainly be the one to use for starting. However, the other reason never to start on the stand is that in traffic at a stop light you will not have the luxury of deploying stands if your engine stalls. I suspect that "stand starting" has become more usual as the miles on the machines drop.
Finally, if you ride two up you need to practice what happens when you stall at traffic light. It should come as second nature. Also, if you ride in a group, once the others start their bikes you should be comfortable following the technique by feel only because you will not be able to hear anything as the others start up. If you ride at a rally or in a group, be prepared to start your stalled Vincent on a public road without the aid of hearing.
David