VML and Canvas provide very different APIs and behave very differently. Explorer Canvas maps the Canvas API onto VML with very poor results. It may be possible to build a library that uses either Canvas or VML but they're so different I almost think it'd be easier to target each separately. Either way, this is a non-trivial pain in the neck.
It's amazing how desperately people want canvas compatibility in IE. It's such a simple API, too. It's hard to imagine that it would take any real resources for Microsoft to create a nice, fast implementation. But, once again they seem to prefer holding the web hostage.
You're right though, it seems unlikely that a plugin will succeed. But, the fact that someone's working on creating one is more proof of how lacking the other workarounds are and how much people want this technology.