(Check out my profile for a link to my site containing more up-to-date reviews and bonus media!)
Unlike the Red Shoulder Document, The Last Red Shoulder feels like a totally necessary part of the Votoms series, and I can't imagine it not being a part of the story. Seriously, it's odd everything in this wasn't in the actual series, as it's pretty important. Regardless, watch Red Shoulder Document first, as this has an important connection to information only shown in that OVA. This OVA takes place between the Woodo and Kunmen arcs (after episode 13), and ideally should be watched between them.
For starters, there are more significant plot developments in this than most single episodes of the Votoms series, and that's a huge plus. We get to see the origins of Ypsilon and learn that he didn't just come out of nowhere for the Kunmen arc. We get to see more insight into Chirico's past and some greater detail on the characters that make it up, as well as some closure that makes the Red Shoulder Document a much more worthwhile watch. The animation isn't as good as the aforementioned OVA, but is still superior to the series. Recommended.