Should chicken be seasoned before vacuum sealing?
There's no benefit to seasoning before you seal your food unless you plan to cook the food in the bag once it's thawed, as in sous vide. Salting or spicing isn't going to make it last longer in the fridge, and it's not going to impart much in the way of flavor either. 

I suppose seasoning and rubbing your spices on before freezing would save you a bit of time later if you had a specific recipe in mind for the chicken, however if you decided to use the chicken for a different purpose you'd have to clean all that off and re-season/spice it if you did.