One of the advantages of SpongeVanilla is that it can be updated much more quickly than SpongeForge, the latter of which relies of Forge to be updated. Once the MCP mappings are released for 1.9, we can get to work updating SpongeVanilla.
At the moment, there’s no ETA on either update, but you can expect SpongeVanilla for 1.9 to be available much sooner than SpongeForge.