Hi and welcome to the forum.
I’m not sure you can break the ASCII VTK files apart into pieces. You might be able to break your structure into pieces and write them out at separate ASCII VTK files though? You might also look into another easy-to-write data format (like OBJ, PLY, STL). The last two can be binary as well, which cuts down on size a lot.
However, if your software already has enough understanding of VTK to write out legacy VTK ASCII files … perhaps try using the library to write a modern VTK format? If you describe your data, we can probably help you out. Your code snippet appears to be Python, which makes that even easier.