I did wonder at first if setting UseDoublePrecisionVertices to True might help here
https://developer.rhino3d.com/api/RhinoCommon/html/P_Rhino_Geometry_Collections_MeshVertexList_UseDoublePrecisionVertices.htm
but it seems that if you do this then retrieve a vertex position using Mesh.Vertices[0], it still returns a Point3f
Maybe someone else from McNeel can clarify how this works. Does it only affect vertices which are added or moved after switching it on? and does Mesh.Vertices[0] then return a Point3d?