With Forge having native support for "core-mods", we found it fitting to allow plugins to provide mixins when using SpongeVanilla. Now, it is possible! Plugins that provide mixins through normal means can have their mixins loaded by SpongeVanilla.
What does this mean really? It means that if a server administrator focuses solely on SpongeVanilla for vanilla features (no intentions of using forge or content adding mods), it's possible now to provide as many modifications to the game as deemed fit. I'm sure the docs will be written up soon on how to take advantage of this system, or perhaps @minecrell might document it.