Depending on how large is large, it will be far easier to replace the seat if worn instead of the complete valve (and for sure cheaper). On small valves, typical 2"-4", it would make little sense to replace the seat as it most likely will be cheaper to replace the complete butterfly valve (if it's a low cost trim, typical cast iron body and 316 stem /disc/buna n seat). We use eccentric butterfly valves for throttling (low pressure / high flow application) and would not recommend the use of vulcanized seats as the seat typically would be replaced each 3-4 years.
Liners versus vucanized.  You first have to ask your self how critical is the system the valve is in.  If the system can not be down long you may want to look at a complete valve to change out. Up and running quickly. But this can also be pricey.  Replaceable liners will allow for you to change out and repair on site. It will also allow you to stock several types of liners depending on what you are pumping.  EPDM, PTFE, BUNA etc.  You can engineer one style body with the different liners.  Much more versitile.  Don't count pennies at first engineering if possible.  Saving a few dollars on cheap valves can cost major dollars down the road.  Liners are not necessairly cheaper also.  I am presently repairing an 8" Fisher B/F with a PTFE liner.

MORE NEWS