Here is what I found:

Minimal Documentation: https://milkv.io/docs/duo/getting-started/cv1800b Datasheet: https://github.com/milkv-duo/hardware

Reading the datasheet, it looks like there is one C906 cpu with 700 Mhz without the the vector extension and one C906 cpu at 1Ghz with rvv 0.7.1. The C906 design has been opensourced and is available here: https://github.com/T-head-Semi/openc906

The C906 supports rv64gc with optional rvv 0.7.1 with a vlen of 128, but a 256 wide ALU.

They list H.264/H.265 support, but I don't think it's a standardized extension.

But see my other comment about using the pre ratification vector extension: https://news.ycombinator.com/item?id=36377439#36378911