If I understand you correctly, then I’ve been told that keeping a GeomVertexWriter across multiple frames is not the intended approach, nor safe.
(See this post for more.)
I’m not hugely familiar with the use of the GeomVertex -Writer/-Reader system in Panda, but looking at your code, I’m wondering whether the problem doesn’t instead lie with your calls to “modifyGeom” and “modifyVertexData” at the end of the “highlight_object” method.
Specifically, I don’t think that either is called for when using a GeomVertexWriter.
Further, looking at the API, I see that “modifyVertexData” is actually meant to return a reference to the geometry for the purposes of modifying it. I wonder whether it wasn’t invalidating your use of the GeomVertexWriter before it…
[edit]
Also, welcome to the forum! I hope that you find your time here to be positive! 
Also also: a quick ping to your user-name, @eakuz, to alert you of my response (hopefully), in case you hadn’t planned on reviewing the thread, given that it was marked as “solved”.