i dont allow vaping in my factory and it would go down as gross misconduct and i would either worn or sack them depending on what machine they where on.
its all covered in there contract and they sign forms when they join to ensure they are aware of it.
from a company point of view, if i had a member of staff needing 12 weeks on the sick i would also be looking for any reason to get rid of him. sounds awful but being a small company (15 staff) i cant carry dead weight.