Can you break Vimpat in half if you have trouble swallowing. Secondly, do you have to wean off it?

No and yes, respectively. Talk to your Dr.!

From Drugs.com:

"Measure liquid medicine with a special dose-measuring spoon or medicine cup. If you do not have a dose-measuring device, ask your pharmacist for one.

Do not stop using Vimpat suddenly, even if you feel fine. Stopping suddenly may cause increased seizures. Follow your doctor's instructions about tapering your dose."

https://www.drugs.com/vimpat.html

With regards to cutting the pill I suggest you ask your pharmacist, but as the site mentions it comes in liquid form as well.