To definitively determine whether you need a new chain, either buy a Park Tools CC-4 chain checker for $15 or measure the chain using a steel ruler as shown on this page:
https://www.sheldonbrown.com/chain-wear.html. If everything is within spec, just clean and lube the chain. If not, replace with an 8-speed chain as suggested above. Do NOT buy a used chain from that guy. He gone done you wrong!
I also agree that you should stick with the Vx derailleur.