As a backup solution, you can still edit vertex animatins by importing/exporting individual frames.
There are a few caveats about that procedure, see
[this] post.
Depending on what kind of vertex animation you want do to, that could be just what you need.
Naturally using a file format for vertex animation would be a lot faster.
I don't know much about .vta file format. I think I might have built an importer for it once in another project, but I'm not sure.
You can try pm-ing me an example of the data you would like to be able to import in openBRF, I'll see.
However, I wonder if it is maybe possible for XSI user to grab some MD3 importer instead.